Demons

 First-Class Devils by Johannot, Tony

Amorphous otherworldly beings with great powers and magical items. They can only inhabit the human world by possessing a corpse. The most common way a demon is summoned is via sacrificial ritual. 

The ritual first calls the demon closer to our world. Once it's at the border the sacrifice is killed. Allowing the demon to possess it and enter our world. Regardless of how powerful the demon is, the body they inhabits still rots. To counteract it they end up needing to consume similar corpses to rebuild their body. This gets worse as demons highly prefer to possess human bodies over anything else. Leading to more dead human to satiate it.

Demons can also be called directly by specific humans, but it's much more rare. If the human's desires something great enough and the demon is able to grant those desires a connection can be made. The demon will then jump into the nearest corpse it can possess. 

While demons are generally powerful (Min HD of 5), they still wouldn't last that long against the full brunt of humans that put their minds to it. So instead they play the long game, manipulating humans to gain followers and power.

Demons have an intense desire to stay in our world. Nobody is entirely sure why they want to be in our world. Whenever asked they always spout out lies. The most common being they're an angel (or the divine being itself) come to bring prosperity to their followings.

The few rare adventurers that have made contact with multiple demons speculate that they seem to just enjoy living in our world. Newly birthed demons look like they take pleasure in their experiencing mundane sensations. They also have an obsession or two they ruthlessly pursue. If it's not being spent on their own protection, they use of all of their resources into the obsession.

When the first demon appeared in the world, it caused a ripple among the existing monsters. Brand new species of monsters that could speak started appearing. Monsters that couldn't speak before suddenly become able to.

This distortion caused the then stable age of humanity to quickly fall into confusion and ruin. Humans on the whole have yet to fully recover from this, many ruins of previous civilizations now dot the landscape. 

Author's Note.
This might fit better for the definition post, but didn't want to radically change it lel. Very much inspired by "You're Doing Demons All Wrong" by Goblin Punch and "Your Demon Lord Doesn't Need That Many Hit Dice" by Against The Wicked City. 

The part of the first demon causing the other monsters in the world to become more dangerous was directly inspired by Jujutsu Kaisen and how when Gojo was born Cursed Spirits became more dangerous.