Kotaku reports in an updated post via word from Randy Pitchford:

Gearbox Software says that Mad Moxxi’s Underdome Riot is “not just new deathmatch areas.” CEO Randy Pitchford notes “This is an ENTIRELY NEW GAME MODE.”

“Think [Halo 3] ODST’s ‘Firefight’ or Gears of War’s ‘Horde Mode.’ Cash, prizes and competition!” Pitchford said in a statement from Gearbox. “It’s like Smash TV in coop FPS, but in the Borderlands.”

Gearbox reps say the new Borderlands downloadable content “expands greatly on the possibilities of combat as well adding a ton of new rules such as vampire mode or low gravity mode…”

MTV MP Blogs reports that the harder arenas might take six hours to complete:

It’s all about this crazy Moxxi character, and she’s running these tournaments. You go in and fight waves and rounds of monsters, and then she starts changing it all up. She throws out what are called ‘Moxxi Maxims,’ these rules that dramatically change the way the game play. She’ll be like, ‘Vampire Rule!’ which makes it so that the player’s health is constantly draining, but every time you get a kill you get a whole bunch of health back. Now all the players are like, ‘Holy crap, I have to play amazingly aggressive,’ so you have to charge in before you basically bleed out. Another example, she’ll go ‘Shotgun Challenge,’ where all shotguns do bonus damage and all other guns take a penalty.”

There’s a small tournament and a big tournament. The small tournament takes probably about an hour, and most people will be able to get through that. The big tournament…one of the things people have been asking for is for something that challenges them, and that’s what the big tournament is. I think the last time I went through it, it took me six hours to get through a big tournament. You can’t save your progress, you have to do it in one sitting, but you can stop, walk away from it and come back [Ed note: You just can't turn the system off]. It’s really hardcore. To get through the big tournament, you’ll have to get some friends who are very good and you’ll all have to plan out what people are going to do and what weapons you’re bringing.”

There are also three screenshots total now, see them all below (click for larger versions)

New York, NY – December 10, 2009 – 2K Games announced today that the second piece of downloadable content for Borderlands™, Mad Moxxi’s Underdome Riot, will be available on December 29, 2009 from the Xbox LIVE® Marketplace for the Xbox 360® video game and entertainment system from Microsoft for 800 Microsoft Points and on January 7, 2010 from the PlayStation®Network for $9.99. The downloadable content will also be available on Windows PC. This explosive piece of action-driven content is in development at Gearbox Software and provides players with more methods of destruction as they tear through the denizens of Pandora.

Mad Moxxi’s Underdome Riot is an intense single-player or cooperative experience that expands the mayhem by adding three new Riot Mode arenas where players will endure an onslaught of Pandora’s baddest enemies. Working together, players will fend off the masses as they fight for fame, honor, and more importantly, loot. Mad Moxxi’s Underdome Riot also adds a new bank feature, which allows players extra storage capacity for when they encounter one of the more than 16 million weapons that Borderlands has to offer; and the ability to acquire two additional skill points as quest rewards, making their characters even more powerful than before.

Featuring extensive co-op integration that seamlessly blends the single-player and multiplayer experiences, Borderlands allows players to freely join or leave each other’s games at any time for collaborative mayhem or the freedom to choose to play in full single-player mode to experience the world on their own. Borderlands also includes a groundbreaking weapon generation system that offers a near infinite variety of weapons to the player. Borderlands combines these fan favorite features with addictive non-stop action to form a breakthrough experience that challenges the conventions of modern shooters and RPGs.

Borderlands™, rated M for Mature by the ESRB, is developed by Gearbox Software and is currently available for the PlayStation® 3 computer entertainment system, the Xbox 360® video game and entertainment system from Microsoft and Windows PC. The Zombie Island of Dr. Ned, the first piece of downloadable content for Borderlands, is currently available for all platforms.

Click Images for larger version.