In Pokémon Colosseum, the final main game battles are fought here. At this time, it is a newly opened Colosseum, where Cipher plans to hold Shadow Pokémonbattles. Nascour arranges a tournament of four Shadow Pokémon Trainers the player must defeat, before challenging the player himself. After Nascour is defeated, the true head of Cipher, Evice, shows up and battles the player using the ultimate Shadow Pokémon, Tyranitar. After being defeated, Evice attempts to escape with a helicopter, but it is shot down by Ho-Oh's Sacred Fire, and Evice and his cronies are arrested by Chief Sherles and Officer Johnson. This tournament can be rechallenged during the post-game to snag any missed Shadow Pokémon.
Spoilers end here.
In Pokémon XD: Gale of Darkness, Realgam Colosseum is now a proper Colosseum, hosting fair tournaments. Over the five years since the events of Pokémon Colosseum, Realgam Colosseum has replaced Phenac Stadium as the preferred Colosseum of the people of Phenac City, causing the latter to be closed down.
The Realgam Colosseum is a glorious stadium built in the center of three mechanical towers. The Colosseum itself is a gigantic, open dome, with bleachers for spectators to watch. There is a giant Poké Ball shaped circle in the center the battlefield. The Colosseum itself is surrounded on almost all sides by vast, complicated networks of large towers of varying sizes.
The order of the Pokémon in a Trainer's team may be somewhat randomized when the player battles them. Therefore, depending on which Pokémon ends up in the last slot of their party, the Trainer's prize money may also vary.