S9E31 : Battling the Enemy Within!

Ash is really excited to get to the Battle Pyramid, but he falls down a hole into a strange ruin. When Team Rocket goes to check it out, they end up falling in as well. As Ash and his friends explore, they set off a trap! They're saved by a man and his Regirock—the man's name is Brandon, and he tells Ash off for messing with the ruins. The ruins, he explains, are where the King of Pokelantis sealed a Ho-Oh. But instead of being careful, Ash gets excited and runs off in search of Ho-Oh. He finds something much more sinister when he awakens the spirit of the King, who possesses him and demands everyone's Pokémon as part of plan to rule the world!