Altars of Sorrow
From Destinypedia, the Destiny wiki
Altars of Sorrow is a guardian-initiated activity that can be triggered as a Public Event in the Sorrow's Harbor on the Moon similar to Escalation Protocol and Blind Well, where a team of Guardians must kill waves of Hive and Nightmares. There are five tiers of increasing difficulty and teams engage a final Nightmare boss at the end of their chosen tier, ending the event.
In reality, the Altars of Sorrows are rituals meant to sacrifice Nightmares for their Darkness essence. This is then utilized for constant resurrections of Zulmak, Instrument of Torment by the remaining leadership of the Hidden Swarm.