the old chateu is a old mansion near eterna city and eterna forest if you have completed the pokedex (not the national one) you can go into the room where there is a tv and go up to it between 8 pm...
Go into a room at midnight where the tv is on and it says something like it looks like there is a Pokemon undre the tv thump it then press yes and you will battle him at lv 15
First go to Route 205. Cross the bridge. Go up then left. Then you go in the forest and you'll see some trees. Use Cut on the trees. Then go inside. Be prepared. There are ghosts in that place. The...