You go down to the third level, annd into the farthest left pit.
At the desert resort
You sell it to the relic maniac, in the Undella mansion.
in the desert resort
In relic castle
at the end of it
It can be found in the Relic Castle in Pokemon Black Or White
fossil in relic castle
Yamask can be found in the Relic Castle.
Relic Castle Catacombs in the wild. You have a 15% of finding it.
Relic Castle after you beat the Elite Four.
it is in the desert near relic castle
its at the very end of relic castle it is VERY hard to find:(