I killed Mom. Not the first time, probably not the last time either, but this time I noticed a large hole in the wall, not like a secret room, but way bigger. Inside I had a choice of loot and then waves upon waves of bosses. And then more reward. It was fun.
What can I do to guarantee/increase my chances of this boss rush happening?
Best Answer
You need to beat Mom in The Depths / Necropolis in less than 20 minutes from the start of the run. Boss Rush will always appear if you do.
Note that it unlocks a different item/achievement for each character you beat Boss Rush with.