Majin Buu, Troublemaker

Whenever you attack, each opponent loses X life and you gain X life, where X is the number of attacking creatures.

If Majin Buu, Troublemaker and a creature named Evil Buu, the Heartless are attacking, and you both own and control them, exile them, then meld them into Kid Buu, Chaotic Evil. It enters the battlefield tapped and attacking.