Gameplay in the coliseums of The Underdome consists of waves and rounds. Each round consists of five waves.
- Starter wave: A mixture of enemies that will be fought in the subsequent rounds.
- Gun wave: Every enemy will have weapons and only regular bandits/brutes or Crimson Lance will spawn.
- Horde wave: Only psychos will spawn with all types (regular, burning, badass, and midget) present.
- Badass wave: Only a few of the enemies will spawn as badass variants, but each will come with a couple of smaller equivalents; bandits are upgraded to brutes, Lance Infantry are upgraded to Engineers etc. An assorted group of Guardians may also spawn.
- Boss wave: A random boss is selected to be fought, as well as a few henchmen. These include Nine-Toes and his skags, Bone Head, Baron Flynt, Hanz and Franz (always without henchmen), Master McCloud, King Wee Wee, Reaver, Sledge, One-Eyed Jack, Taylor Kobb.
- At the end of waves one through four, Moxxi will drop two types of Supply Packages. One of ammo consisting of 18 revolver, 18 sniper, 24 shotgun, 54 repeater, 54 combat rifle, 72 submachine gun rounds, and 2 grenades, the other being Mega Healing Vials. The beginning of the supply drop is signaled by a brown box attached to a parachute and Fireworks. These drops will land all over the arena and only last a few seconds before disappearing.
- Some enemies will drop Super instaheath vials that stay until the round is finished.
- A random selection of weapons will appear at the bottom of Moxxi's tower once the boss and his henchmen are defeated. Characters will have ten seconds to choose before the weapons disappear.
- It appears that not all characters see the same selection of items with some only being visible to certain characters.
- While it is possible to get loot from enemies in the Underdome, it is extremely rare.
- Mordecai's Swipe skill will generate additional item drops, although it is usually money or health with weapons also as a rare drop.
Every round will increase in difficulty and the number of enemies to kill increases after five rounds. More enemies will teleport into the arena over time during each wave. If all the members of a party are defeated, then the whole party starts at wave 1 of the previous round. Any subsequent losses will not take the party to the previous round.
|1||-4%||-6%||-8%||none||Starting out easy|
|3||0%||0%||0%||1||Welcome to average|
|5||+4%||+6%||+8%||2||Professional grade tournament||Last round of the Small Tournament|
|6||+6%||+9%||+12%||2||Are you man enough?|
|7||+8%||+12%||+16%||2||I eat round 7 for breakfast, do you?|
|8||+10%||+15%||+20%||2||Kill kill kill|
|9||+12%||+18%||+24%||2||Brick's Mama got past this round|
|10||+14%||+21%||+28%||3||You rock, I'll roll|
|11||+16%||+24%||+32%||3||These guys take a lot of hits|
|12||+18%||+27%||+36%||3||Lock & Load|
|13||+20%||+30%||+40%||3||I'm not dead yet|
|14||+22%||+33%||+44%||3||Bigger, stronger, faster|
|15||+24%||+36%||+48%||4||What, did they just add another rule?|
|16||+26%||+39%||+52%||4||Suck it down|
|17||+28%||+42%||+56%||4||Better hope your guns deal a lot of damage|
|18||+30%||+45%||+60%||4||Game over man, game over|
|19||+32%||+48%||+64%||4||They deal how much damage?|
|20||+34%||+51%||+68%||4||Grade A - Badass||Last round of the Big Tournament|
Rules, also referred to as Moxxi's Maxims will be introduced at:
- One modifier after the 1st round
- Two at the start of the 5th round
- Three at the start of the 10th round
- All four at the start of the 15th round
Examples are listed below.
|*weapon* Challenge||A random weapon type is chosen (for example, Shotguns). That type does more damage and everything else does less. Eridian weapons don't seem to be included.|
|Body Shots||Critical hits do less damage.|
|Close Combat||Weapons will be less accurate but do more damage.|
|Elemental Challenge||Elemental attacks will work better, normal attacks will do worse.|
|Headshot||Critical hits do more damage, everything else does less.|
|Beef Cake||Enemies take less damage from attacks.|
|Dead Aim||Enemies are more accurate.|
|Dodgeball||Enemies actively dodge attacks.|
|Ironhide||Enemies have better shields.|
|Kick Ass||Enemies will deal more damage.|
|Loaded||Enemies have larger magazines.|
|Overclocked||Enemy shields regenerate faster.|
|Regeneration||Enemies regenerate health.|
|Spastic||Enemies move and reload faster.|
|Weapon Masters||Enemies spawn with higher quality weapons.|
|Get Some Air||Low gravity. (Formerly impossible jumps may be tried to reach new sniper spots.)|
|High Speed||Characters and enemies move faster.|
|Naked||Your character has no shields, but will slowly regenerate health.|
|Vampire||Your character's health constantly degenerates but can be refilled by killing an enemy. If your character gains a second wind, both health and shields will be fully replenished.|
If a character dies and fails to get a second wind in a multiplayer game, they respawn in the penalty box on top of Moxxi's tower. The character will be stuck up there, but can still help the rest of the team by shooting down on enemies and throwing grenades. Characters in the penalty box will not be targeted by enemies, their health will not drop below one point and will respawn at the bottom of the tower when the wave is finished or the rest of the party dies.
If a solo character dies, they respawn in the penalty box for a short while before respawning at the foot of the tower and restarts the tournament from the first wave of the previous round.
Unlike respawns in regular gameplay, no money is lost on death.
Characters will be rewarded after the completion of each round. After defeating the boss, weapons a few levels below the host's level will spawn under the character spawn point. These weapons are usually of good quality, and Eridian weapons have a fair chance to be given.
Having a multiplayer game increases the quality and amount of weapons after each round.
- Main article: Underdome glitches