Eons ago, there was a war among gods that came close to destroying humanity and the mortal realm. Gods depend on humanity, and their destruction would inevitably hurt them as well. At the conclusion of this war, a pact was signed among the gods to prevent this cataclysm from happening again. Gods agreed to leave the mortal world and humans to their own devices. They enacted a barrier around the realm to keep outside forces from entering.
The mortal world is protected by a barrier covered in runes that are designed to keep out other entities. There are many dangers to the mortal world. Other realms exist, such as the fae and demon realms, each with supernatural creatures hostile to humanity. Invasions can occur, in which a being from these realms tries to take over the mortal world and enslave or wipe out humanity. They enter the realm through tears in reality created by sorcerers who summon them. This requires a complex ritual that must meet certain parameters, such as location, time, materials, etc. Although it is a difficult spell to cast, the summoners survive the process. These tears can be used by an opportunistic creature to enter the world and widen the opening, allowing others to break through. Sometimes the invaders are just too powerful or numerous for mortals to handle them, and can only be matched by a deity of rival power.
There exists an ancient spell that is used in times of need. Functionaries of the gods (priests, holy warriors, shamans, etc) can summon a powerful deity to fight on behalf of humans and help them fend off these supernatural creatures. Unfortunately, this ritual is so powerful that it takes the lives of the users, and requires hundreds of people to work. They sacrifice themselves to power the spell and bring a god into the world, and are seen as noble heroes who sacrificed themselves to protect the planet.
Why would a god require the lives of hundreds of worshippers to cross over into our world when other supernatural creatures simply require a traditional summoning?