Your adventure in Pokémon Colosseum begins in Phenac City, a bustling hub that serves as your introduction to the game's world and mechanics. Upon arrival, you'll witness a confrontation involving a mysterious figure and the first instance of Shadow Pokémon being used. This event will set you on your path to investigate Cipher and their activities. You'll receive your starter Pokémon, Eevee, and your first mission will be to track down Cipher agents operating within the city.
Phenac City is where you'll learn the basics of movement, interaction, and your first battles. You'll encounter various trainers and Cipher Peons, providing opportunities to practice your battling skills and, crucially, to snag your first Shadow Pokémon. Pay attention to the environment and any dialogue from NPCs, as they often contain hints about where to go next or what to do. Securing your initial Shadow Pokémon and understanding the snagging process here is paramount for building your early-game team.