You're gonna get a special one because I actually have to do this under the line
AHEM: Anyone in the Into the Darkness we March campaign do not proceed. This will contain spoilers.
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So this is Key/The Key/Keystone! They're a powerful entity in my Into the Darkness We March campaign setting and a being who's sole interest is in collecting knowledge.
Or so it seems.
Keystone, the players will eventually discover, is technically the reason the world got very fucked up/everyone's actions turning them into monsters/twisting their souls into beasts of destruction and sin.
Currently, the belief is that something happened to the main goddess Phera, or perhaps one of her daughters (Danera, Vodona, and Temlene). In reality it's because Keystone is a forgotten entity to the masses, only a select few even know of their existence.
The main antagonist/BBEG is going to be an old follower of Keystone's who is responsible for corrupting the gods themselves to take their power for their own.
Keystone knows this and isn't going to tell anyone, even their own followers in the party.
Parading around as an Entity and Collector of sacred knowledge and secrets, they're really and Entity of Time, Ruin, and Secrets.
The reason I say Entity here is that due to everyone forgetting (which was part of Keystone's plan) they lost their deity status (from lack of prayer). Still being a very powerful creature with their own pocket dimension, they just became a patron for warlocks, though is picky about who they choose.
So the big final battle for the players is gonna have a few stages:
First they fight the reoccurring antagonist
Then time stops and they come back in an avatar of Keystone state
At the end it very much will come to light that Keystone orchestrated this entire event because they were bored and wanted to see what the mortal plane could do to solve this mystery. How long would it take them to learn the truth? Which, they're honestly disappointed seeing as they had to lay a heavy hand into the situation.
Keystone isn't all bad though!! Seeing that the mortals were outright killing those of animal nature (fearing they were easier to become beasts) Keystone reached out to some mutual friends in other planes to harbor them until the world entered a period where they'd be able to more easily hide! In doing so, they saved many races from genocide and allowed them to continue to flourish.