Dragon Ball Z, Vol. 25: Last Hero Standing! (Dragon Ball Z, #25)

The djinn Boo is now purely evil and kills six billion people. Only the heroes in Kami-sama's sky-palace survive. There, Goten and Trunks merge into Gotenks, hoping that their combined power will be enough of a match for Boo. On a faraway planet, Gohan and Goku prepare to return to the fight.