This Arena is ruled by the Fight King and appeared in the episode "Morituri Te Salutamus". When Finn and Jake reached the door, the Fight King tempted them to come in. Once they entered the doors were magically sealed and they were forced to fight the Fight King's Gladiator Ghosts. Each round they survived, they were given a palm frond and sent underground to rest. When they were done resting, they called out to Fight King, and he returned them for the next round, using his magic sword to imbue Finn with bloodlust. Once Finn beat all the ghosts, the Fight King ordered him to kill Jake. However, Finn easily overcame the Fight King's influence and turned the tables on the villain.
The arena appears to be an ancient colosseum imbued with some of the power of the Fight King, because it was guarded by a moonlight barrier and sealed its entrances after Finn and Jake walked in. It has a large underground chamber where the remnants of gladiators rest. When the Fight King died, the arena fell apart and followed the ghosts.
Finn later surmised that the Fight King used this same process on all the pairs of combatants before they became Gladiator Ghosts. After Finn broke the Fight King's sword and destroyed him, the Gladiator Ghosts left, and the arena magically disintegrated and floated away.