From Destinypedia, the Destiny wiki
During the course of Bad Neighbors the Guardian finds that a conflict between the Hive and Fallen on Titan, who had arrived after following the survivors of the City to Titan, has escalated and the Fallen are now going to build a bomb to kill the Hive. However the bomb will cause too much damage, risking the structural integrity of the entire Rig, forcing the player to intervene and intercept the Fallen in the process of creating the bomb. Ghost then decides however that this bomb could still be useful if used away from the Rig, against the Hive nests at Tidal Anchor. Ghost uses the Fallen's bomb components to craft his own bomb, which the player then uses to destroy a Hive "Resonator" device, luring out the Hive in Tidal Anchor responsible for its construction, and killing them all.