Hell-burbia is a clapped out sprawl of buildings built for claustrophobic split-level fighting; Angelic Ruins, with its bleached alien architecture and more open aspects, can seem a bit like an expanded take on Halo's Snowblind at times; and The Gully is an oppressively vertical space where snipers can earn their keep beneath a series of slow-spinning wind turbines.
Everything has the dirty-fairylights trappings of an in-bred circus to it, and all three maps are stitched together from pieces you'll recognise from the main game, delivered in the stylishly ramshackle manner you've likely come to associate with Borderlands by this point.
The whole thing's charmingly merciless, too. After the first round in each coliseum, Moxxi's Maxims kick in, each altering the way the game plays in a manner that will be familiar to anyone who's ever found any of Halo's skulls.
In one wave Moxxi might power up a certain weapon class while downgrading all the others, while the next time she may fiddle with Pandora's gravity, boost enemy health bars or - most chaotically - speed up everybody's movement. As rounds mount, more of the maxims switch on at once, and they provide the perfect jolt of chaos to bring the game's predictable timetable of pain to life.
There's an important caveat, however: if you're a lonely Borderlander, you're going to find Moxxi's challenges tough to solo. Rules like the penalty box (if killed, you sit out the rest of the round in a lofty prison you can still provide sniping support from) are built for co-op, and the sheer ferocity of the waves themselves suggests Gearbox is unsubtly prodding you out of the house to get some friends.
Even played as a group of four this can be a steep challenge, and you'll want to put aside the best part of an hour to race through each coliseum in the first - and easiest - tournament. Attempting the second tournament by yourself, with enemies scaling to your level and no one to help you when you're downed, will be genuinely punishing.
But it will be enjoyably punishing too. By dialling back the experience grind and loot collection, Mad Moxxi not only comprehensively proves that Borderlands is capable of providing sharp thrills by means of its combat alone, it also offers you a chance to stop the ceaseless progression for a few hours and take a good long look at what you've actually become.
Playing as a Siren, I've been happy to discover that, for once, I've been investing skill points almost wisely, with a phase-walk capable of rebuilding practically all of my health in one go before I rocket back out of the astral plane in a devastating burst of electrical energy.
Borderlands' latest DLC has shown me the combat rhythm I've naturally settled into: the regular back and forth between whittling away at enemies with SMGs before disappearing in a puff of magic, to either slink off and let my shield charge up again, or pop up behind an unfortunate bruiser in order to blast chunks out of their neck with a shotgun.
If the Underdome had "merely" been a Horde mode, that may have been fine, but it's a lot more besides. It's a glimpse into the internal machinery of Pandora, a rare chance to understand where you've been, and where you want to go next.