Obligatory MTG lore warning, Duskmourn specific lore as well...

User is a Planeswalker- the reason you came to Duskmourn is up to you, perhaps it was an accident (cough, Winter, cough). But since you came here, Valgavoth has decided you can’t leave.

He wants to learn everything about the other planes, how else could he learn how to create doors to get access to them? So in the massive moth monster’s infinite mercy, he’s decided to keep you alive. At least, until he’s learned everything he thinks he needs.