After the player's team rescues Medicham from Wish Cave and clears Stormy Sea, Gengar will visit the Team Base to ask to be escorted up Mt. Freeze. Once at the peak, Ninetales will tell Gengar to go to Murky Cave. This unlocks the dungeon, though Gengar must be escorted through it before it is possible to visit alone.
Gengar will give the player the Mobile Scarf as a reward. The morning after this dungeon is completed for the first time, Gardevoir will appear in Pokémon Square, asking to join the rescue team.