"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

The Dungeon is one of the three possible rewards for completing a quest, but I wouldn't choose it; the hero inside isn't that strong.
I personally dealt with it. I just compared what would happen in one case versus the other, and since demons weren't coming to me through the portal (still don't know why), I didn't need their help either.