Western Cave

Final Floor: B20F
Unlock Method: Defeat Ho-Oh in Mt. Faraway
Initial Level: -
Maximum Teammates: 3
Items: Yes
Check Map: No
Opponents Able to Join: Yes

After you have encountered Ho-Oh in Mt. Faraway, you will see Charizard and Blastoise have gone off on an adventure in a cave to the west where the most powerful Pokémon is said to exist. This opens up Western Cave for you. Upon completing the dungeon, on B20F you will find the Legendary Pokémon Mewtwo. It will Mega Evolve half way through the battle. Defeat it and it will automatically recruit and give you the Cryptic Cave Camp