Which episode did ash's monfermo evolve?

Ash's Monferno evolved into Infernape in the episode titled "The Battle Finale of Legend!" This episode is part of the Diamond and Pearl series, specifically the 21st episode of the Sinnoh League. The evolution occurs during a climactic battle against Tobias, where Monferno evolves after a moment of intense determination and fighting spirit. Infernape then showcases its new powers in the battle.