Sorry, I've been busy with other things.
Polyamory involves three or more people in a consenting, non-monogamous relationship.
As for the plot: three adventurers who grew up together decide to go on quests for fame and fortune. While all three were skilled and strong, one adventure into a certain dungeon proved to be too much. While two of them (my character(s) and your character) were able to escape, one (your character) was unable to and fell deeper into the dungeon. While the other two adventurers thought the third had died, he survived. Not only that he grew stronger and became something of a ruthless"demon lord," descending into a madness and believing his friends had betrayed him.