Dragon Ball Z, Vol. 25 by Akira Toriyama

Shorn of his last vestiges of goodness, the djinn Boo is now pure, undiluted evil! With a single wave of his hand he kills six billion people, leaving only the heroes in Kami-sama's sky palace alive...but for how long? Inside the palace, Goten and Trunks merge into Gotenks, the only being in the world who might match Boo in raw power. But luckily there's more than one world. On a faraway planet, Gohan and Goku are preparing for their turn to fight...to save the devastated earth before the planet itself is blown away!