S02E52: The Truth About the Mist

The unexpected person, turned from Chrome, turns out to be Mukuro Rokudo. A series of chaotic illusions battle starts. During the fight, Tsuna experiences flashbacks of Mukuro on how he sacrificed himself for Ken and Chikusa to escape. Mukuro ends up being the winner, almost killing Mammon, who escaped from the battle field with his reserved power. Mukuro fainted after the battle, turning himself back to Chrome.