There is a veil which separates our reality from a parallel dimension that contains Eldritch abominations. An predatory empire has discovered that they can harness the power of this realm by summoning demons across the barrier. This is done through the use of human sacrifices.
The more powerful the demon, the more sacrifices are needed. The sacrifice of one human being would be equivalent to one measly familiar, while the death of thousands would yield a more powerful daemon. Weaker daemons are subjugated by their masters and used as servants. Stronger daemons cannot be subjugated in this way due to being more powerful. Instead, they are used for specific purposes before being sent back to their realm (specific spells, reveal dark secrets or information, create items, etc).
This empire preys on its neighbors to provide the fuel it needs for their rituals. This includes the weaker states surrounding it as well as more powerful Nations further out. As such, thousands of individuals are taken from these surrounding nations as a form of tribute. However, an empire in our world known as the Aztecs also operated under this mantra, taking slaves from other nations through conquest. What eventually happened was that all of their enemies banded together with the help of conquistadors to overthrow them. The Aztecs had pissed off so many people with their constant need of sacrifices that their weaker nations destroyed them through strength of numbers.
Surrounding nations of this demon worshiping empire aren't likely to take the kidnapping, torture, and sacrifice of thousands of their people for the sake of this empire's survival lightly. You would think that this empire can simply call upon their demons to fight and defend them, but it is not that simple. Demon summoning requires many valuable and hard to get materials, which often can be used only once. Rituals are time consuming and expensive, and individuals can't just call down an army of demons to back them up. training and skill are required for summoning. Certain parameters must also be met, such as specific locations, dates, and environment. The economic factors alone would put a large amount of strain on this empire, not to mention other factors. Therefore, warfare is not a long term solution in most cases.
This empire needs to continue taking large amounts of tribute from its neighbors while at the same time preventing them from taking sides against them. How can they avoid this scenario and maintain the status quo?