In Pokémon Black , you find Volcarona in Relic Castle , deep in the post-game area that opens after you beat the Elite Four and the Champion. It’s a static encounter at the very bottom of the castle, where you can interact with it to battle and catch it.

How to reach it

  1. Go to Desert Resort.
  2. Enter Relic Castle and keep going down through the sand pit rooms.
  3. After the post-game opens the blocked path, follow the deeper route through the castle.
  4. You’ll eventually reach Volcarona’s chamber at the end.

Notes

  • In Pokémon Black and White, Volcarona is Level 70.
  • There’s also a Larvesta egg you can get in Route 18 , and Larvesta evolves into Volcarona at level 59.

TL;DR

Volcarona is found in Relic Castle , but only after the main story is finished.