To unlock an epilogue, you must unlock and read the character's five stories, defeat all region bosses in the order the game gives you to unlock Morden's third phase, and defeat Morden.

You may choose to abandon a run when all stories are unlocked, then read the stories at the Barracks before running for epilogues. In case you forget, "Defeat Morden" for the stories themselves doesn't care about phase 3.

You may unlock up to 3 epilogues at a time i.e. of the members in your party.