It's a King Krogmo Boss Rush Arena!
Costs crowns to enter. Probably a few thousand. Why? You gotta have stakes. Prizes are claimed when you die, based on how many bosses you defeated. (Alternatively, one may leave once they reach a terminal to claim prizes and quit the run.) Boss Brawl is always in Tier 3, and always on Elite difficulty. This means that lower tier bosses are a little beefed up (particularly the tutorial ones).