You have to beat team Plasma at Dragonspiral tower. Then, go to the castle and walk, but not run, along the sides of the quicksand until you get to the far left. The girl there will give you a fossil. then head down and complete the maze.
After you beat the Elite Four theres a pileup of sand you can walk over to a door.
Volcarona is inside Relic Castle.
there is a backpacker in relic castle who will give you either a plume fossil (which is revived into an archen) or a cover fossil (which is revived into a tirtouga)
You can only take a certain amount of steps before a i think hurracane? happens.
you can either catch one at the bottom of relic castle or you can hatch a larvesta egg that is given to you by a man in a house on route 18. if you do choose the latter, it evolves at lvl 59.
His name is Darmanitan and you can awake him with a RageCandyBar given by Professor Juniper during the story
you have to get to the bottom
relic castle
Relic Castle in the desert
Volcarona is inside Relic Castle.
Relic castle
It can be found in the Relic Castle in Pokemon Black Or White
It contains the element plate items, as well as Relic items, which can be sold to the relic maniac in the Undella mansion. The item at the bottom is the Relic Crown, the most expensive.
It's west of Desert Resort
one is in the relic castle on route 18
there is a backpacker in relic castle who will give you either a plume fossil (which is revived into an archen) or a cover fossil (which is revived into a tirtouga)
in terms of levels, the strongest pokemon in the wild is a lvl 70 volcarona in one of the bottom rooms in relic castle after u have beaten the main story once
it is in the desert near relic castle