For context you decide wether or not you want to challenge a boss 2x stronger than the main one.
Has a chance of being rare at minimun, and legendary at maximun.
The boss you face is also faster than the original mini-bosses you normally fight.
and has a chance of the boss being a main-boss from each level, mainly occurs if the room you fight in was a mini-boss and has a chance of spawning that challenger chest in.