When you first get to Relic Castle you're pretty limited. You only have the first floor. So, progress onwards in the game until you can actually go through it.
My suggestion is beating the Elite Four and then coming back.
you have to go to nacrene city to get either the dark stone that contains zekrom or the white stone that contains reshiram
You sell it to the relic maniac, in the Undella mansion.
at the end of it
The Relic Statues in both Pokemon Black and White are acually Darmanitans. But these Darmanitans have the secret ability Zen Mode which will change it's form and stats when its HP is low. You can wake them up with the RareCandyBar that Prof. Juniper will give you when you enter.
Zekrom (Pokemon White) and Reshiram (Pokemon Black).
There are no pokemon after pokemon blACK and white but there will be more pokemon games in the future.
It can be found in the Relic Castle in Pokemon Black Or White
it is in the desert near relic castle
At the desert resort
You sell it to the relic maniac, in the Undella mansion.
With a sun stone. a Sun Stone can be obtained in Nimbasa City and Relic Castle.
in the desert resort
at the end of it
In relic castle
fossil in relic castle
At one point in the plot, you will be told to go there. Only then will he move and allow you access.
Yamask can be found in the Relic Castle.
Route 14, route 18, the dreamyard, relic castle, the cold storage, and the charge stone cave