Digimon World 4

The game's protagonists are members of the “D.S.G. (Digital Security Guard)”. Based on Digital Monster X-Evolution, it is immediately revealed that a computer virus known as the “X-virus” is spreading quickly and is infecting many Digimon. “The Yamato Server” has disappeared, and a new server known as “The Doom Server” has taken its place. They are sent to the first area of the game named Death Valley to search for Chief Leomon. When it is completed the player finds out that The Doom Server may in fact be The Yamato Server. The player is then sent to destroy the “Doom Dome”. This is where the first real boss appears, Apocalymon. The player is then sent to Dry Land to stop the X-Virus spreading and must defeat MaloMyotismon. They are then sent to the Venom Jungle to stop the Dread Note from launching and must then defeat Lucemon. They are then sent to the final area of the game Machine Pit to destroy the final boss Mecha Rogue X.

Source: TheGamesDB