This place is like a haunted house. As you travel through the house, the configuration of the lamps and couches that block your path will change. There's only one place to go when you first arrive, the stairs leading to the basement on the west side.
Make your way to the other side of the room. There's a Spell Tag half way through. There is one more item on the east side, but you can't get it quite yet. Use the east stairs leading back to the first floor.
Entering the nearby room leads to a Backpacker.
Going to the second floor using the eastern stairs makes a ghost appear and then disappear. Continue to the opposite side of the first floor and enter the room to find a Full Heal. Use the western basement stairs to go downstairs again.
Make your way to the east end of the basement. This time around you can pick up the Dusk Stone, which will evolve some Dark-type Pokemon. Use the eastern stairs to go back to the first floor. Now you have full access to the second floor.
The right room has a Rare Candy. Climbing the middle stairs leads to another ghost encounter. Enter the middle room to get the Lunar Wing. It's an item that will come in handy later to catch a special Pokemon. Exit the room and head to Reversal Mountain.