When elder evils stir, the world groans; when they awaken, the world weeps.
Buried in the deepest bowels of the Underdark, hidden in the farthest reaches of the multiverse, or lost in the gulfs between realities are terrible things that exist only to destroy or horribly remake creation. So mighty are these ancient beings that even the gods think twice about standing against them. Mortals who are aware of their existence viciously suppress that knowledge and destroy any who would serve such things. Even if an elder evil can be forced back to whence it came, its mere presence changes the world forever. In short, it is a campaign ender.