The Old Chateau is an abandoned building in Eterna Forest that you can reach once you get the Forest Badge from Eterna Gym.
Get an Item
Outside of the Old Chateau, there is an Ether to the right of the building.
The only pokémon that you can encounter in Old Chateau is Gastly.
Explore the Chateau
If you go into the door next to the statue on the ground floor, you reach a large dining hall and see someone float to the left. Go to the left and check near the upper-right trash can to find an Antidote.
If you go upstairs from the main room, and go into the room on the left, you find an Old Gateau.
From where you got the Old Gateau, exit the room, then go north through the upper door.
You reach a hallway with five doors. In the leftmost room, you find a Dread Plate. In the rightmost room, you find TM90 Substitute.