Derelict Mansion Boss Encounter Bug Report

Still bugged after all patches. Amazing work.

I encountered this bug today. Went back to kill all the monsters until the bosses were remaining. Afterwards, I walked back to the boss area and the doors turned into the "Arena" entrance. The boss fight activated once I entered the Arena. It's possible it might have something to do with killing all of the Rares on the map to get the boss room to activate. Who knows.