From Bulbapedia, the community-driven Pokémon encyclopedia.
Darkness Ridge (Japanese: くらやみとうげ Darkness Ridge) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It becomes available once the player has recruited Absol. It is in the Mountain Range area, and has the capacity of eleven Pokémon. The Pokémon here are predominately Ghost and Dark types, and most are recruitable in Darknight Relic.
Darkness Ridge is an area bordered by dark woods with mysterious glowing lights and many rocks.
This mountainous area may be inhabited by mischievous Pokémon who can suddenly turn the area dark to cause trouble.