|"Dark Evolution! Skull Greymon"
Colosseum (コロシアム Koroshiamu) is a huge soccer arena in Server Continent.
The Colosseum is the location of the Crest of Sincerity. The Chosen Children arrives at the Colosseum in search of Jou's Crest, as suggested by his flashing Tag. They are then attacked by a Greymon under Etemon's control, and gets trapped under an electric cage disguised as a goal post. The Crest of Sincerity turns out to be under the ground of the cage. Agumon turns into Skull Greymon through Dark Evolution here.
The Chosen Children found the Colosseum in Episode 21, "Dark Evolution! Skull Greymon", close to an oasis in the desert. Kido Jo's Tag resonated with it, letting him know that his Crest was in it, which made the Chosen Children enter it. Inside the Colosseum, they found a football court, one of its goals had Jo's Tag react stronger than ever. Takenouchi Sora also found a football, and asked the rest if they wanted to play. While the others agreed to it, Yagami Taichi was against it. Regardless, the match wouldn't happen as Etemon showed himself as a hologram. The Chosen Children and their Digimon then ran towards the goal Jo was close to to protect themselves, but Agumon could not get there due to eating too much, and Taichi ran towards him, which saved them from the trap that got everyone else, trapping them inside the goal's net, which Etemon explained had a high-voltage current running through it. Taichi then demanded Etemon let everyone free, but Etemon merely pointed out he couldn't fight him as he was too far, but he had prepared a special guest: a Greymon. To fight it, Taichi had Agumon evolve into Greymon. However, the enemy Greymon would win the match due to Taichi's Greymon's sluggish movements, caused by eating too much. Taichi tried to force Greymon to evolve so he could win, but his Crest of Courage would not react.
The trapped Chosen Children then fell into some ruins in the Colosseum basement. Jo's Tag stopped reacting as strongly, meaning they had gotten farther from his Crest. The Chosen Children then decided to head to the surface to help Taichi and Greymon, as well as find Jo's Crest. As they continued, Jo's Tag had stronger reactions, letting them know they were on the right path. However, it led them to a blocked path, forcing them to take a detour. Said detour would lead them to what seemed to be a deadend, but Izumi Koshiro realized the wall that blocked their path was similar to the one that became the Crest of Courage. Indeed, much like that one, it then glowed and shrunk, turning into Jo's Crest of Sincerity, and the Chosen Children were then able to return to the surface.
In the Colosseum proper, Taichi still was trying to force Greymon to evolve. Ishida Yamato wanted to have the rest back Taichi up, but Taichi stopped him, as that was his chance to get Greymon to evolve. To do so, he made himself a target of the enemy Greymon, which Etemon was fine with. This finally succeeded in evolving Greymon, but it triggered a Dark Evolution, so he became Skull Greymon, who then destroyed the enemy Greymon, a part of the Dark Network and a part of the Colosseum with a single Ground Zero. However, Skull Greymon was now running berserk, and was a risk even to Taichi himself, forcing the other Partner Digimon to fight him.
After the battle, Skull Greymon ran out of energy and devolved to Koromon, who knew he had done horrible things and felt he had disappointed everyone. However, Yamato reassured him he had done nothing wrong, it was all Taichi's fault. Taichi accepted responsibility and asked for forgiveness, realizing he had felt like he had to fight by himself after obtaining the Crest of Courage.
The Chosen Children return to the Colosseum in Sub-Episode 10, "DigiOne Grand Prix", which happens after Episode 23, "The Fairy! Piccolomon". The Colosseum's entrance at this time was full of Digimon, such as a Flymon and an Orgemon arguing with each other, a Vegiemon, a Cockatrimon, a Meramon who trained for a year for the event and was sure it would win, an Unimon and a Gerbemon; due to the namesake event, and a Gazimon quickly entered the Chosen Children into it. Said Gazimon introduced the event as a once-per-year event which he, who did not work for Etemon, produced, with an amazing prize for the winner.
Entering the Colosseum, which had a Drimogemon, a Vademon, a Gizamon, a Mechanorimon, a Mojyamon, a Kuwagamon, an Andromon, a Scumon, an Anomalocarimon, a Monzaemon, and a Tankmon for an audience; the Gazimon producer explained the rules to the Chosen Children: three battles with no breaks in-between, and they would receive a "fabulous" prize if they won. The first fight was against a Mechanorimon and a Guardromon; the second against a Drimogemon, a Monochromon ★ and a Shellmon ★; and a Meramon, a Yukidarumon and an Elecmon. The Chosen Children emerged victorious, and Gazimon gave the victory to "the Agumon team", as well as their prize, a Lifeup.
The Chosen Children returned to the Colosseum in Sub-Episode 12, "Orgemon's Bone", which happens after Episode 30, "Shining Wings! Garudamon". There, they found Orgemon in the entrance to the Colosseum. They were naturally wary of him, however, he pretended he was not that Orgemon. The Chosen Children let it pass, and Takaishi Takeru asked him why he was there, to which he replied that he wanted to get a new Hone Konbou as his old one was all banged up and he needed to use a Skull Greymon bone for it, to Taichi's shock. Orgemon then asked for help and left for the underground of the Colosseum. However, Tachikawa Mimi was unwilling to trust him, though Yamato convinced her to. Once they arrived at the underground, Orgemon recruited the Chosen Children as "bone hunters", warning them to do exactly as he says as SkullGreymon is a troublesome Digimon; Orgemon joins the party at this point. Deeper in the underground, they found a huge door, which Takeru and Patamon opened up as they were curious to what was behind it, which turned out to be a Skull Greymon. Wanting to defend his little brother, Yamato decided to fight the Skull Greymon despite Orgemon's objections.
After defeating the Skull Greymon, Orgemon found a bone he could use. He also praised Yamato for standing up to Skull Greymon and gave him a Defend DigiPiece, which allows the equipped Digimon to automatically take hits for other Digimon with low HP, then left, with Yamato wondering if they should've let him go.