Murderbrawl XXXI is part of the natural story (part of Act 3).
At the end, choose either option: one will net you Killbane's mask, and the other, the Apoca-Fists (otherwise available for $100,000 I'm told).
The mask will set people you compliment or taunt on fire, and the Apoca-Fists are a melee weapon that makes your target explode.
I went with the latter.