Relic Castle Volcarona .

Thought to be an embodiment of the sun, it appeared during a bitterly cold winter and saved Pokémon from freezing.

In Pokémon Black & White, the player gains access to a new part of the Relic Castle in the Desert Resort,
after receiving the National Dex Expansion. This Volcarona is encountered at Level 70.
[Unsetting bitflags 0348 and 0810 allows it to be encountered again]

In Pokémon Black 2 & White 2, the player gains access to the collapsed portion of Relic Castle via the cave near the Pokémon World Tournament. The initial Volcarona is encountered at Level 35.
If this Volcarona was defeated, it would return at Level 65 after the player defeats the Elite Four.
[Setting the event flag at 0404 means Lvl 35 was captured, and setting the flag 0928 means Lvl 65 was captured]