The Brotherhood of the Apocalypse is an organisation from megasean3000's fanfic Xiaolin Showdown/Yu-Gi-Oh Crossover 3 that has one goal in mind: Steal the Seven Millennium Items and Three Egyptian God Cards to dominate the Earth.
Gorondorlin, the leader of the group, was once best friends with Master Fung and Master Valing, they even played Duel Monsters in their younger days in a Grand Tournament, similar to the one played in Xiaolin Showdown/Yu-Gi-Oh Crossover 3 except played with Ancient Magic found in Egypt. But when Master Fung defeats him with the three Legendary Knights, Gorondorlin sought ways to defeat him. Eventually, he came across an Ancient Artifact that houses an evil spirit from Ancient Egypt, that possesses him to set out it's dark designs 5,000 years past.
That's when Gorondorlin found Jarvis, Barto, Paul and Antonio, four powerful duellists who each fall under the same spell. Now the Brotherhood of the Apocalypse was formed. After much research, they eventually found out that Bakura was in possession of the Millennium Eye, after sealing the dark spirit to the Shadow Realm, they recovered their first Millennium Item. Using the eye's Dark magic, they created Duel Disks housing the Eye's Shadow Magic, that turns Monsters and Spells real.
Once created, Gorondorlin hosts a Grand Tournament that gathers all famous duelists, including Yugi Muto, Seto Kaiba, Joey Wheeler and the Four Xiaolin Warriors together, and plays in groups with a best two out of three duels. After realising Gorondorlin's plans, the Duelists quickly set out to stop Gorondorlin. However all who have faced the dark group have all failed, Marik Ishtar lost, Yugi lost (After giving his Millennium Puzzle to Raimundo.) and the Rival-Warriors lost. It is then that Raimundo faced Gorondorlin in the final duel, it was a worthy face-off, with Gorondorlin controlling all three God-Cards and wielding Six out of Seven Millennium Items. But using the Winged Kuriboh Kimiko owned, Raimundo turns the tables on Gorondorlin and wins, releasing his soul of the dark spirit and saving all those who lost to the Brotherhood.
Gorondorlin apoligizes for the evils he has inflicted, and hands over the Millennium Items, which are returned to their original owners, and the three Egyptian God Cards.