Fanfic!

Title: The End of Days
Author: Korsriddare
Characters: Eirika, Seth, other TSS characters
Rating: K+
Summary: An evil more primal than the Demon King has been awakened. As Magvel faces destruction, Eirika and Seth struggle to survive.
Spoiler(s): References and some endgame spoilers of Fire Emblem: The Sacred Stones.

http://www.fanfiction.net/s/5681651/1/The_End_of_Days

Decided to make it multi-chapter after all.