There is no need to have rebellious nobles declare themselves kings when the empire weakens, though that happened in a number of cases. it is quite possible to normally have kingdoms within the empire.
In history there are many examples of dependent kingdoms, maybe more than independent kingdoms. In the 19th and early 20th centuries newly independent European countries became kingdoms, and thus it became usual to think of a king as the sovereign of an independent country.
Many people would said that since today only 32 independent sovereign states are kingdoms (16 of them with Elizabeth II as their queen), there are only 16 kings in the world today. But that is not entirely accurate.
For example, the head of state of the United Arab Emirates is a president. But the country consists of seven emirates ruled by hereditary emirs who form the ruling council. The president and the prime minister are elected by the council, but the Emir of Abu Dhabi has always been elected president and the Emir of Dubai has always been elected prime minister. So some people would count the emirs in the UAE as sort of kings within a federation of kingdoms.
Malaysia consists of 13 states and 3 territories. Nine of the states are monarchies, with one raja, seven sultans, and one Yang di-Pertuan Besar as heads of state. The nine monarchs elect the head of state of Malaysia, the Yang di-Pertuan Agong, from among themselves for a five year term, but by agreement the position is rotated from sultan to sultan in a predetermined order. The Yang di-Pertuan Agong is often called the king, but it makes as much sense to call him the king of kings and the monarchs of the states kings.
Wikipedia has a list of current constituent monarchs, hereditary heads of state of groups within various countries.
This is a list of currently reigning constituent monarchs, including traditional rulers and governing constitutional monarchs. Each monarch listed below reigns over a legally recognised dominion, but in most cases possess little or no sovereign governing power. Their titles, however, are recognised by the state. Entries are listed beside their respective dominions, and are grouped by country.
Even if only a small percentage of those traditional rulers should be considered kings, that still increases the number of kings at the present significantly.
In medieval Ireland, there were many small polities called Tuaths. The ruler of a Tuath was called a Ri, or king. Thus there were an unspecified number of kings and kingdoms in medieval Ireland - I have read 90 kingdoms, and 150 kingdoms in another source, so I wonder if anyone has a complete list.
Wikipedia lists about 57 kingdoms in the "early Christian" period.
The Island of Ireland has an area of 84,421 square kilometers or 32,595 square miles. The population of medieval Ireland at various times may have varied from about 500,000 to about 1,000,000. So if there were about 50 to 200 kingdoms at any one time in medieval Ireland the average kingdom would have had an average area of about 422.105 to 1,688.42 square kilometers, or about 162.975 to 651.9 square miles, and an average population of about 2,500 to 20,000 people.
So average medieval Irish kingdoms might be bigger than Barbados and smaller than Coromos, with populations larger than the Vatican City and smaller than San Marino.
Some kings were overlords of other tuaths besides their own. A common title for them was Ruiri or over king. They could be called second level kings or kings of kings, but many Christians have a strange reluctance to use the title of king of kings.
Ireland was always divided into areas that are called provinces in English, but were called "fifths" in medieval Gaelic. Due to the ambitions of kings, there were often more than five fifths in medieval Ireland. The king of a fifth, or province, was the overlord of all kings and overkings in his fifth, and was often called a Ri ruirech or king of overkings. It would be logical to call a Ri ruirech a third level king or a king of kings of kings.
And for centuries there was a high king of all Ireland or Ard ri. It would be logical to call the Ard ri a fourth level king, or a king of kings of kings of kings, instead.
And some medieval Irish writers claimed that the High King was subordinate to the Holy Roman Emperor, thus making the emperor equivalent to a fifth level king or a king of king of kings of kings of kings.
The ancient Roman republic and empire had many kingdoms under its authority at various times.
Tiberius Claudius Cogidubnus was a client king in southern Britain after the Roman conquest in 43 AD. A fragmentary inscription was reconstructed as giving Cogidubnus the title of King and Imperial Legate in Britain, but it is now reconstructed to give him the title of Great King of the Britons, the rank of great king being justified by the emperor granting him lands outside his original kingdom to rule, according to one theory. Thus we see that not just kings but great kings could be clients of the Roman Empire.
King Tigranes II The Great of Armenia conquered many neighboring lands and proclaimed himself King of Kings about 85 BC. After being defeated in 68-66 BC by Lucullus and Pompey the Great, Tigranes became a client of the Roman Republic. I believe that Tigranes II and his son Artavasdes II continued to use the title of of King of kings until 34 BC, which would make the Roman consuls in that period the republican equivalents of third level kings or kings of kings of kings.
I am not sure what titles were used by succeeding Armenian monarchs, king, great king, or king of kings. Most of the later Armenian monarchs were subordinate to either Rome, or Persia, or sometimes to both at once.
In 36 and 34 BC the Roman Triumvir Mark Anthony granted many lands to Queen Cleopatra of Egypt. In the Donations of Alexandria in 34 BC Mark Anthony proclaimed Cleopatra and her son Caesarion, the queen and king of Egypt, Queen of Kings and King of Kings.
Mark Anthony made his young children with Cleopatra monarchs. Anthony appointed Alexander Helios King of Armenia, Media, and (unconquered) Parthia, his twin Cleopatra Selene II Queen of Cyrenaica and Libya, and Ptolemy Philadelphus King of Syria and Cilicia.
Thus Mark Anthony showed that a Roman triumvir had authority to appoint kings and kings of kings, and thus was equivalent to at least a third level king or a king of kings of kings.
But since the Parthian monarchs were now using the title of king of kings, it is possible that Mark Anthony might have promoted Alexander Helios to King of Kings when and if Parthia was conquered. If Cleopatra remained the overlord of Alexander Helios that would in turn make her the equivalent of a third level queen, a queen of kings of kings, and that in turn would have made Mark Anthony at least as high as a fourth level king or a king of kings of kings of kings.
The Borsporan Kingdom, founded in about 480 BC, became a roman client state at lest as early as 8 BC, and lasted until AD 341 or later, before being conquered by the Huns. It ruled parts of the Crimea and the area around the Sea of Azov. Theodore Momensen, in The Provinces of the Roman Empire 1885, 1886, stated that one of the Bosporan kings, a client of the Roman Empire, used the title of king of kings.
Septimius Odaenathus was made the ras (lord) of Palmyra, a city state in the Roman Province of Syria, in the 240s. In 252 Persian King of Kings Shapor I began a long series of invasions of Roman provinces. In 260 Shapor defeated and captured Emperor Valerian at Edessa and raided the eastern provinces. Fulvius Macrianus proclaimed himself and his sons Quietus and Macrianus Minor emperors and sought to overthrow emperor Gallienus, son of Valerian.
Odaenathus may have been proclaimed king of Palmyra at this time. Odaenathus defeated Shapor and drove him out of Roman territory. Gallienus defeated and killed Macrianus and Macrianus Minor in a battle in the Balkans. Odaenatus then defeated Quietus and his followers and took control over much of the eastern provinces, and was rewarded by Gallienus with titles and honors.
In 263 Odaenatus took the title of King of Kings of the East and had his son Hairan I crowned co-King of Kings.
In the Roman empire's hierarchical system, a vassal king usage of the King of Kings title did not indicate that he is a peer of the emperor or that the vassalage ties were cut.
Odaenatus and Hairan I were assassinated in 267. Odaenatus's widow Queen of Kings Zenobia became regent for their young son King of Kings Vaballathus. Palmyra invaded Egypt and Asia Minor in 270, and Vaballathus and Zenobia took the titles of Augustus and Augusta, emperor and empress, in 271. Emperor Aurelian defeated them and conquered Palmyra in 272.
In 337 Emperor Constantine I planned a war against the Sassanians. He appointed his nephew and son-in-law Flavius Hannibalianus Rex Regnum et Ponticarum Gentum ("King of Kings and of the Pontic People"). Pontus is the name of the coastal region along the Black Sea in northeastern Asia Minor. But Constantine died in May 337 and Hannibalianus was murdered in September by the sons of Constantine.
Ancient Armenia was mostly conquered by Persia in 428. Ashot I the Great, Prince of Princes of Armenia, was recognized as King of Armenia by the Caliph Al Mu'tamid in 884. King Ashot II the Iron, though more or less subordinate to the Byzantine emperor and/or the Caliph, became King of Kings about 922. A later Armenian king of kings granted kingdoms to several family members. Armenia was conquered by the Byzantine empire in 1045.
in Georgia, Prince Adarnase IV of Iberia became king of Georgia in 888. His descendant Bagrat III became the first king of all Georgia in 1008. The title of King of Kings was used by later medieval Georgian monarchs, but I don't know when they started using it. King David IV the Builder (reigned 1089 to 1125) is said by some sources to have been the first to use the title of King of Kings and also the first to refuse offers of Byzantine titles of honor, thus claiming independence from the Byzantine empire.
But David's abdicated father George II apparently lived until about 1112 and is described once as king of kings. Thus George II may have both used the title of king of kings and also have accepted Byzantine titles, acknowledging their
And here it is stated that Gurgen II, king of part of Georgia, used the title of King of Kings during his reign (994-1008).
So it is uncertain for how long the Georgian rulers both used the title of King of Kings and accepted titles from the Byzantine emperors, thus also acknowledging the over lordship of the Byzantine emperors.
Anyway, these examples show that it is quite possible for emperors to have not only vassal kings but also vassal great kings and vassal kings of kings.
Some people might be surprised by these examples of kings of kings being subordinate to emperors, because some people think that the title of king of kings is equal to emperor, or even higher.
That is because of various biblical verses:
Which in his times he shall shew, who is the blessed and only Potentate, the King of kings, and Lord of lords;
These shall make war with the Lamb, and the Lamb shall overcome them: for he is Lord of lords, and King of kings: and they that are with him are called, and chosen, and faithful.
And he hath on his vesture and on his thigh a name written, King Of Kings, And Lord Of Lords..
So some people would think that it would be blasphemous for a mortal to use the title of King of kings since that title has been used to describe God.
But Christians also call God king. And the most common name for God in the Bible is The Lord. So by the reasoning that makes calling someone a king of kings blasphemous, calling someone a lord or a king would also be blasphemous. So if a fantasy writer wants to avoid having characters using the title of king of kings for fear of sounding blasphemous, he must also give up having characters using the titles of king and lord.
And if The Lord is sometimes used to mean God, then what does lord mean in Lord of Lords? Does it mean ordinary low ranking human lord, or does it mean god or even God? Thus one has to wonder what King of Kings and Lord of Lords means.
Does King of Kings and Lord of Lords mean:
King of Kings and (mortal) Lord of (mortal) Lords
King of Kings and (divine) Lord of (mortal) Lords
King of Kings and (mortal) Lord of (divine) Lords
King of Kings and God of Gods
These parts of the New Testament were probably written about AD 50 to 150 in the Middle East. And what were the most powerful realms in the Middle East in those days? The Roman Empire and the Parthian or Persian Empire under the Arsacid Dynasty. The Arsacid monarchs used the title of king of kings by that time.
So the writers of Timothy and Revelation didn't create a new title of king of kings for God. Instead they used a title which they must have known was used by contemporary mortal rulers and had been used for centuries. Which may be why they used the unique form of King of Kings and Lord of Lords.
You may have heard about Ethiopian emperors. But actually King Victor Emmanuel III of Italy is the only person I am certain used the title of Emperor of Ethiopia, from 1936 to 1943. The Wikipedia article "Emperor of Ethiopia" says that the title was actually Negusa nagast "King of Kings". Important officials and tributary monarchs often had the title of Negus or king.
The use of the title may go back to King Sembrouthes of Axum about AD 250, or to about 296, but was used by the time of Ezana (320s-c.360) the first christian monarch. Thus all the Christian monarchs of Ethiopia for over 1,600 years used the title of king of kings. As well as many Christian monarchs of Armenia and Georgia. In medieval Ireland, the Christian kings of the provinces used the title of Ri ruirech or king of overkings, which can be considered even higher than king of kings.
The opinion that king of kings is equal to emperor is not very sound.
The Persian rulers of the Achaemenid Dynasty ruled the mightiest realm the world had yet seen, the first true empire in history. Their title was "The Great King, the King of Kings, the King of Lands and Peoples, the King of the World (or the Universe)". They were kings of kings who actually were equivalent to emperors.
After the conquest by Alexander the Great and the break up of his realm, the Arsacid Dynasty of Parthia revolted against the Seleucid Dynasty and gradually conquered Iran and neighboring regions from the Seleucids. Their kings took the titles of great king and later king of kings. The Arsacids directly ruled the central provinces of their realm, but there were at least a dozen vassal kings of outer provinces. Later Iranian tradition claimed there were 90 kingdoms in Iran in that era.
In AD 224 Ardashir I, king of Fars (Persia proper), revolted against Artabanus V, King of Kings and founded the Sassanid Dynasty. Ardashir defeated Artabanus and took over the Parthian Empire, and annexed many of the small vassal kingdoms. Ardshir called himself King of Kings of Iran. His son Shapor I called himself King of Kings of Iran and of Non Iran.
The Arabs conquered the Sassanid Empire in the 7th century AD. Ismail I (1487-1524) conquered Iran from 1502 to 1510 and proclaimed himself Shahanshah, King of Kings. Various dynasties ruled Iran and parts of other countries as kings of kings until 1979.
The Persian Kings of Kings of the Achaemenid Dynasty, and to a lesser degree those of the Arsacid and Sassanid Dynasties, and to a still lesser degree those of the dynasties after 1500, can be considered emperor equivalents.
The Mongol Empire in the 13th century was the largest empire that had ever existed up to that time. The supreme ruler is usually called the Great Khan in English. But the title was actually Yekhe Khagan, Great Khagan, and Khagan means khan of khans, or more or less king of kings. So the Mongol ruler actually used the title of Great King of Kings. And certainly the Mongol ruler, who had many subordinate rulers, was equivalent to an emperor.
And there have been other kings of kings who were more or less equivalent to emperors. But most kings of kings throughout history have been much lower than emperors.
In the British Empire of India, about 60 percent of the land was directly ruled by the central government, and about 40 percent was ruled by various native states. The more powerful Muslim rulers mostly used the title of Nawab, while the more powerful Hindu rulers used the title of raja (king), and the most important of them used the title of maharaja (great king). But some of the most important maharajas used an additional and even higher title, maharajadhiraja (great king of kings or king of great kings).
A wealthy Bengali landowner, Sir Jatindramohan Tagore (1831-1908), was granted the title of maharaja in 1877 without being given a kingdom to rule, and was promoted to maharaja-bahadur in 1890 and the title was made hereditary in 1891. Bahadur in a title makes it one step higher. His brother Sourindra Mohun Tagore was made a raja in 1890.
Man Singh I, Raja of Amber (1550-1614) was a vassal of the Mughal Empire and a Mughal general. In a biography, he is sometimes called king of kings. And that should be a title of honor granted by the Mughal Padishah.
I believe that the highest titles for Hindus in the Mughal Empire included, besides lower titles:
Raja - king.
Raja Bahadur - illustrious king.
Maharaja - great king.
Maharaja Bahadur - illustrious great king.
Sawal Maharaja Bahadur - elevated illustrious great king.
Rajadhiraja - king of kings.
Rajadhiraja Bahadur - illustrious king of kings.
Maharajadhiraja - great king of kings or king of great kings.
Maharajadhiraja Bahadur - illustrious great king of kings or illustrious king of great kings.
The Mughal ruler used titles like Padishah (lord or master of kings), Khagan (khan of khans), and Shahanshah Al-Sultanat Al-Hindiyyah Al-Mughaliyyah (king of kings of the Sultanate of India and the Mughals).
The Chinese title of a king was wang, and huangdi is commonly translated as emperor. During the Han Dynasty (202 BC-AD 9 and AD 25 to 220) the imperial government directly administered various commanderies in China, but there were also a number of kingdoms within China. I believe that there were about 50 kingdoms in the Han Dynasty at various times. Emperors of later dynasties down to the Qing (1644-1912) sometimes appointed some of their sons kings.
Many writers sort of chicken out about calling the wangs in imperial China kings, preferring to call them princes.
The huangdi was supposedly the rightful ruler of all the world, but conquering other countries was hard, and convincing them to become tributaries was easier. Many states, probably about a hundred, not directly ruled by the Chinese empire were at one time or another part of the tributary system and acknowledged the huangdi as their real or nominal overlord, and some of those states were kingdoms or higher realms.
In Medieval western Europe the Carolingian Empire was divided into up to ten different kingdoms at different times in the 9th century, and whoever was emperor claimed to be the overlord of the various kingdoms.
King Harold the Fair haired conquered the 20 or 30 kingdoms and states in Norway about 872, but for several generations afterwards the King of all Norway was the overlord of various sub kingdoms ruled by real or alleged descendants of sons of Harold the Fair Haired.
In 962, Otto the Great, King of the medium sized kingdom of Italy or Lombardy and the vast kingdom of Germany, was crowned emperor, founding the Holy Roman Empire. All later emperors were kings of Germany and Lombardy, and after 1032 kings of Arles or Burgundy also.
In the early centuries the Holy Roman Emperors were often the overlords of the kingdoms of Denmark, Hungary, and Poland (though Poland was usually ruled by dukes when subject to the empire.
In 1085, Emperor Henry IV make Duke Vratislaus II of Bohemia (died 1092) a king for life. In 1158 Emperor Frederick I made Duke Vladislaus II (c. 1110-1174) a king. I am uncertain if this was supposed to be another life kingship or hereditary. Vladislaus II abdicated in favor of his son Frederick in 1172, and Fredrick and his successors used the title of Duke. Duke Ottokar I (c. 1155-1230) managed to get appointed hereditary king of Bohemia by Philip King of the Romans in 1198, Emperor Otto IV in 1203, and Emperor Frederick II in 1212. Every king of Bohemia up to 1806 was a vassal of the emperor, though by chance many of them were also emperor.
Emperor Henry VI married Constance, heiress of the kingdom of Sicily, though Tancred claimed the crown in 1189. Henry VI forced the captured King Richard I of England to pay him a king's ransom, and some sources say Richard had to become the emperor's vassal and pay annual tribute for all his lands including England. Henry VI conquered Sicily in 1194. Byzantine emperor Alexios III Agnelos agreed to pay Henry VI tribute. The kings of Cyprus and (lesser) Armenia became Henry's vassals. And one source claimed the Caliph of the Almoravids paid tribute to Emperor Henry.
In the 19th century, when newly independent nations became kingdoms, the reverse happened in 1871 with the formation of the German empire. The kingdoms of Prussia, Bavaria, Wurtemburg, and Saxony became parts of the German empire.
in the 19th century, the Hungarians wanted Hungary to be recognized as an independent country, but the kings of Hungary and Emperors of Austria usually wanted to make Hungary part of Austria. In 1867 The Empire of Austria and the Kingdom of Hungary became the dual monarchy of Austria-Hungary, independent of each other except for a few common institutions.
Meanwhile, a region of Croatia became the administrative Kingdom of Slavonia in 1745, part of the larger Kingdom of Croatia which was more or less (depending on Croatian or Hungarian opinions) part of the Kingdom of Hungary, which the Austrian emperors sometimes incorporated into the Austrian Empire depending on how centralizing their government was at the moment. Thus the Austrian Emperor was sort of his own vassal several times over. He could have called himself king of Slavonia, king of kings of Croatia, and king of kings of kings of Hungary. In 1868 Slavonia was merged with Croatia into the joint kingdom of Croatia-Slavonia.
Anyway, these are some examples of kingdoms and higher realms inside of empires and other realms.
Also see my answer to this question: