Immortality vs Decay.
Hundred years ago. There's a cataclysmic event that engulf the world. Releasing evil back to the surface of the world. There's a war. And we lost.
We fled across the sea when evil can't chase us. Or so we thought.
Today the daylights are getting shorter. The sun rise late today, and who knows? Maybe one day it will stop rising entirely.
And during the night. The mist come back, bringing fear, madness and evil back to the inhabitants of the land.
You're eternals. Tethered to life by magical ritual, you're the first and last line humanity's defender against evil.