Right by the entrance to the city, there is a Chansey and a poke vendor to help you refresh and restock for the upcoming gym. Computers for switching Pokemon can be found in most homes.
The Erebus City Gym is the official Gym of Erebus City, whose Gym Leader, Adam, specializes mainly in Dark-type Pokémon. The gym has several laser puzzles that you have to rotate mirrors to focus the laser into the receiver to unlock the way forward.
Move Tutor
After beating the gym, the artist of the far left can teach your Pokemon several Dark-type moves.
Requires beating the 8th gym to use, allows return to the Torren region through Route 5. Note this is a one way trip until the endgame, so finish any business you have left to do in the Dream Realm before you go.