Rex rgis of Ter
Me I'm a Creator
I'm pretty sure that's Cthulhu in the lake. Founding the Octopus Overlords spawns Cthulhu into the world, to travel around destroying cities and giving the "Insane" promotion to every unit that gets within 3 tiles of him. He can only be killed by using the Ashen Veil's WorldSpell to summon another Old One from the Far realm to battle him, which completely consumes the city that casts it and spawns eldritch horrors randomly throughout the entire map.
I wish!