From Destinypedia, the Destiny wiki
As the heroes of the last safe city turned their attention to frontiers beyond the protection of the Vanguard, new Nightmares have emerged from the shadows of our long-forgotten Moon. Called forth by the haunting visions from her tormented past, Eris Morn has returned and unleashed something that once slumbered beneath the lunar surface – a long dormant power even she cannot control.
This expansion is about the Young Wolf returning to the moon to defeat a new evil at the request of Eris Morn. This darkness has supposedly resurrected old demons of player's past such as Crota, the Eater of Hope, Dominus Ghaul, Skolas, Kell of Kells, Taniks, the Scarred, and possibly more. Eris tasks you to put them and whatever evil is out there out of their misery. The Guardians venture deep under the lunar surface and face the new threat, not knowing, what they might set loose...