The Theatre of Pain is a fictional organization created by Marvel Comics for their Marvel 2099 line-up. The group is analogous to the Brotherhood of Evil Mutants or the Hellfire Club in that it was one of the main antagonistic organizations against the X-Men 2099. They were a very well known and feared group that had many resources at their disposal. It was mentioned that the Theatre had more resources than some nations did at the time.
The founder of the organization was a diabolical (and demonic appearing) mutant named Brimstone Love who claimed to be centuries old. The group consisted of a few sadomasochistic individuals who would record the physical and telepathic torture of prisoners and then resell it to clientele who had similar fetishes. In the course of the groups history—before the storyline begins in X-Men 2099 #1- Controller 13 finds and captures a young woman named La Lunatica—a mutant with the ability to trigger painful memories in others—and forces her into his employ so that he can record the traumatic events and resell them to the Theater's clientele.
Following the formation of the X-Men by Cerebra, Brimstone Love notices a spike in mutant activity and has Controller 13 send La Lunatica (later known as Luna for short) out to investigate.[1] She captures Bloodhawk for Controller 13 and he is so delighted by the exquisite images of pain from the man's mind that he sends Luna out to get more mutants. She comes upon Skullfire, Krystalin and Meanstreak and captures the trio to bring back to the Theatre. When Luna attempts to access Skullfire's memories of when he accidentally killed his girlfriend, he releases a huge burst of energy, freeing him and his friends—as well as Luna—from their bonds.
Luna escapes from Brimstone Love and allies herself with the X-Men. However, Brimstone Love does not want to allow his best agent to go free, so he pursues her. However, instead of capturing Luna, Brimstone Love sets his eye on Xi'an because of his newly rediscovered evil nature. He later offers Xi'an a position in the Theater of Pain, and shockingly, Xi'an accepts.
But before one can join the Theatre of Pain they must go through an initiation first. For Xi'an it is to face three people from his past, which he passes by the Theatre's standards. Then he is buried alive. While inside of the coffin, he refuses help from the good side of his persona, effectively entering him into the ranks of the Theater.
Xi'an's friends in the X-Men, along with Junkpile and his S.H.I.E.L.D. agents, infiltrate the Theater and attempt to free him from Brimstone Love's clutches. Everyone is re-captured but Luna breaks free and liberates the other X-Men. They begin to attack Brimstone Love and, unexpectedly, so does Xi'an who wants control of the Theater all to himself. In a weakened state, Brimstone Love teleports away and he and the Theatre are never heard from again before the series closes at issue #35.[2]