I've heard people passing through mention some kind of underground fights pitting creatures of the waste against one another for sport. And when I say "underground", that's their word I'm using, not my own.
The Thorn is an underground settlement located by the Westside South Entrance. The R in the sign is pointing to the sewer, letting you know that is the main entrance. The main interest of The Thorn is the arena, where you can bet on the outcome of fights between wasteland creatures, or fight in the arena yourself. Access to The Thorn is denied from 1:00 am to 6:00 am.
The Thorn is a large arena surrounded by a system of raised catwalks for the guards and spectators, and surrounding the sides are cages holding the wasteland creatures grown & kept by Red Lucy for Arena battles.
If you like, you can set up custom fights by talking to Red Lucy. You get to choose what creature and how many of it will fight in red vs. blue team. Since the creatures are expensive to breed, keep, and maintain, you will have to pay for the fight. If the team you bet on wins you get your caps back plus a little extra. To initiate the fights simply talk to Red Lucy and select "Let's talk about The Thorn." and ask "What's fighting in the next match?"
You can also tell Red Lucy that you want to pit yourself against the creatures. The creatures you can fight are fire geckos, giant mantises, radscorpions, night stalkers, cazadores and deathclaws. You can choose what and how many creatures you want to fight. The screen will fade to black and you will be moved to the arena cage. The winnings are dependent on what it is you fight and the number you are facing. The most you can fight is three deathclaws at once. You must bring her the eggs of the animal you wish to fight, before you can fight them in the arena.
The cage areas of the Thorn may be used to collect experience by shooting the imprisoned animals through the cage doors. Although you will not be able to loot their bodies, you risk nothing in the fights. It's basically a shooting range - all you lose is your ammunition. Be wary, though, that the caged Deathclaws still have a considerable damage threshold, making it useful to employ armor piercing ammo.
After the completion of Bleed Me Dry, the Courier can sleep with Red Lucy regardless of gender.
Using explosive weapons with big blast radius is not recommended in the arena, because the splash damage might cause the spectators to turn hostile towards you, although it is a possible strategy if you do not have the perk Splash Damage or use smaller-scale weapons.
Entering the arena when a fight is in progress will cause all the guards in The Thorn to become hostile and attack. However, if you attack and kill them, this does not cause the spectators or Red Lucy to become hostile.
With ED-E as a companion, the robot may help you out and fire at any enemies you are fighting against in the arena. This will not result in hostilities from guards or spectators.
Remember to talk to your follower(s) after you fight and select the 'Follow Me' option (if you aren't going to fight again). When you go into the arena your companion(s) is/are automatically set to 'Wait Here' mode because they cannot follow you into the arena.
Boone's Spotter perk may help you to see the creatures you're fighting in the darkness at the other side of The Thorn arena.
Occasionally, Fiends may be pitted against radscorpions, giant radscorpions, nightstalkers, or cazadores.
The terminal in Red Lucy's chamber has a hidden option to turn off the power in the cages. It was meant to start a quest where you release the animals to wreak havoc in The Thorn and Westside, but the quest was cut and the option disabled in the final release of the game.
pcED-E will get stuck in the pipelines of the ceiling, requiring you to send him home or to the Lucky 38.[verified]
pc Crash-to-Desktops are very common in this area, especially while fighting in the Arena.[verified]
xbox360pcps3 After defeating any animals in the Arena, the doors open allowing you to leave. A second or two later all the gates that just opened for you to leave instantly seal and the only way out is through the side of the Arena, which itself is sealed unless this glitch occurs. This can easily cause you to get caught in between the two doors leading into the Arena with no way out except the console. This doesn't happen every single time but it's extremely common.[verified]
xbox360 When fighting any number of cazadores or deathclaws, the creatures may turn on each other, stinging/clawing each other to death. The doors will not open after they are dead, leaving you trapped in the arena. Also, the fight never ends when fighting one deathclaw, the doors do not open, and you are trapped forcing a reload.[verified]
xbox360pcps3 After choosing multiple participants for a fight (3 Rad Scorpions on one team, for example) the cage doors will open and the participants will walk out toward the middle of the arena but they will not all stop in the correct "zone" to initiate the fight. As a result, both teams will just sit there and no fight will take place. Early attempts at leaving/returning/fast traveling/letting a few days pass did not seem to reset this. If you attempt to "move" the participants by jumping down and nudging them, or if you try shooting them, the inhabitants of The Thorn will open fire on you for interfering. This makes it impossible for any further fights to take place. To fix either kill one of the teams, exit The Thorn, wait a few (game) hours then return, things should be normal. Or if Lucy is facing the arena, to her right and behind her there is an area you can sneak where one team is visible. With a silenced weapon, kill that team and the match will end. You can exit the pit from the stairs located behind Lucy. If you kill the team undetected, things will be normal. A Stealth Boy may help in this.[verified]
xbox360 When entering the arena and going to fight the door may open but you cannot get through it. An invisible wall will block your path. You can however get through it by crouching then jumping or just walk forward and keep jumping.[verified]
xbox360 Choosing to fight in the arena may cause any current followers to go missing.[verified]
ps3xbox360 When using a Plasma or laser weapon type, and disintegrating enemies into goo/ash, the goo/ash will not disappear immediately like corpses would.[verified]
pcps3xbox360 It is easily possible to walk through the gate before entering the battle area.[verified]
xbox360pcps3 Sometimes the enemies will back out through the door they came from (before the gate) before it closes or even if it has already closed. This prevents the battle from being completed, and so stops the courier from leaving. If you're lucky enough, the enemy may reveal itself through the door by walking into it (a giant radscorpion's claws, for example), and you may be able to get a few cheap shots at it. It's advised to save before the battles, and not to use explosives on creatures from a distance before they get free as this can throw them back through the door or possibly hit spectators, which will turn them hostile. Though to avoid this altogether the player can hug the right wall of the arena and go around the column (next to the exit gate) to approach them so that they will be too close when engaged in combat and will not run away.[verified]
pcps3xbox360 After asking Red Lucy to fight and she telling you to go to the cage and your screen going black, rather than reloading in the arena below, you may simply reappear in the same spot, with Lucy in a permanent bug state refusing to talk to you.[verified]
ps3 When two creatures start to fight they can disappear thus resulting in Red Lucy refusing talking to you, plus you lose the bet.[verified]
pcps3 If you kill the animal(s) you are fighting before they exit their gate, the match will not end, and you will be trapped in the pit. Even noclipping through the exit gate, Red Lucy will be stuck saying "The crowds are waiting for you, stranger."[verified]
pc When two radscorpions are chosen to fight, the animation on their attacks will lock, only tilting back and forth, and neither of them will die, even after zero health.[verified]
ps3 Before you start a fight between 3 giant radscorpions, one will sometimes spawn in your gate but will not be hostile until the gate opens, also possible with deathclaws.[verified]
xbox360 When talking to Red Lucy on the stairs closest to the main entrance, especially following a match against creatures, your partner can get stuck as if you told them to wait. When you tell them to follow you again, they run down into the arena and start running against a flaming barrel.[verified]
xbox360 After completing Bleed Me Dry and thus collecting all of the eggs required, the Strip can become inaccessible. The player can no longer enter The Strip, as a "Requires Key" message under the usual "Enter The Strip" message is present. This can be fixed if you have completed the quest I Put a Spell on You in Camp McCarran or if you have a reputation higher than neutral with the NCR and ride the monorail from Camp McCarran to the NCR Embassy on The Strip. Traveling to Freeside without fast-travel will fix the bugged Stip door, no longer needing a key. However, if you have not passed the credit check for the Securitrons they will attack you once in Freeside. This can be done as many times as needed.[verified]
ps3 When visiting The Thorn's cages, do not attempt to kill caged creatures using melee or unarmed attacks and V.A.T.S. Otherwise, you may end up inside the cage, with no way to exit except by reverting to an earlier save.[verified]
ps3 When you place a bet on a fight they don't fight with each other, this leaves you unable to talk to Red Lucy and you can't win the bet.[verified]
xbox360 When fighting 3 deathclaws, they may get stuck behind the door and the match will not end.[verified]
xbox360 If you fight three deathclaws and win, all the doors may remain locked and will say "Requires key" and you will be unable to leave the arena.[verified]
xbox360 If you fight three radscorpions and win, all the doors may remain locked and will say "Requires key" and you will be unable to leave the arena. This leaves no option but to reload a previous save each time.[verified]
pcxbox360ps3 When fighting three deathclaws, one or more deathclaws may spawn in the player's cage but will not attack until provoked.[verified]
ps3 Sometimes the creatures in matches may spawn in a non-hostile state, making the matches easy wins.[verified]
ps3 If you fall into the ring, without a fight happening, Red Lucy may attack you.[verified]
ps3 It is possible to 'push' the guards into the arena. Most of the time they will 'float' back up to their place or walk through the gates when a battle starts. The enemies will pay no attention to them.[verified]
ps3 Sometimes after completing Bleed Me Dry and engaging in sex with Red Lucy, she will not do it again, but simply repeat the phrase, "Lay down with me, My Hunter."[verified]
ps3pc When the guards in The Thorn become hostile, Red Lucy may not attack you, although her name will appear in red as she is still hostile towards you. If you attack her first, she may retaliate.[verified]
ps3xbox360 Sometimes creatures you're fighting will turn around and walk into the door they entered from and you can kill them easily because they will not be able to fight back.[verified]
ps3 When fighting three night stalkers, the match will never end because they will always be stuck behind the gate.[verified]
pc In the halls with the animal cages, if the courier attacks the creatures inside the cage, then leaves and comes back, the creatures will no longer be trapped in the cages.[verified]
ps3 If the PC kills the spectators or guards of The Thorn and moves their bodies into the arena, the spectators/guards will respawn after a few days where their bodies were moved to. Therefore the spectators/guards can spawn and become stuck in the arena.[verified]
xbox360 Sometimes, when entering the Arena to fight, you will be unable to leave after killing the enemies. This can be fixed by saving and reloading the file.[verified]
xbox360 When in the arena, if you talk to Red Lucy and ask what fight is next and she tells you something and cazadores they both might not attack each other. When you talk to Lucy she tells to wait til the fight is over.[verified]
pc Saving in the Thorn can sometimes result in the game getting stuck on the initial loading screen after clicking continue on the main title upon startup. Loading your last save before inside The Thorn, then loading the most current one should fix this. [verified]
pcps3 ED-E will attack the creatures in the arena when the Courier is fighting them, and does so without any negative implications. [verified]
xbox360 If you ask to fight and attack Red Lucy after the dialogue closes and the screen starts to blacken, she will become hostile, and then neutral again after the fight commences. [verification overdue]