you go to the Dessert Resort and by the "pokemon statues" there's a weird little square and you walk into it and wala you're there!
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In relic castle
you have to get to the bottom
At one point in the plot, you will be told to go there. Only then will he move and allow you access.
You can get Volcarona by catching it at Relic Castle or making your Larvesta (if you have one) evolve into Volcarona starting from level 59.
They let you pick from two different ones inside of relic castle. Ones a seaturtle and the other is a dinosaur bird.