Altars of Sorrow

Altars of Sorrow is a player-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 as a final tier, ending the event.

History
The Altars of Sorrow were first discovered upon the raising of the Scarlet Keep, during the Vanguard's investigation. They are rituals meant to sacrifice Nightmares for their Darkness essence. They were set up by the remaining leadership of the Hidden Swarm to resurrect Zulmak, Instrument of Torment. However, the transfer of energy caused seismic activity, which prompted Eris Morn to send a Guardian fireteam to investigate the depths of the Keep. The Guardians managed to defeat Zulmak and thwart the Swarm's plans, but the Altars remained active, allowing the sisters to continue attempting the resurrection.

Gameplay
The Altars of Sorrow are an escalating public event with five tiers, plus one final tier. Every tier, two adjacent altars are activated, one as the starting point, and one as the end point. The main objective is to stop Ultra Nightmares from going from one altar to the other, sacrificing themselves. The Nightmares will be accompanied by hordes of Hive, and all enemies will only fight you briefly, or shoot on the run, preferring to go towards the sacrificial altars.

Every tier begins with a 1 minute countdown. 1 second is gained by killing a Hive enemy, and 10 seconds are gained by killing a Nightmare. Letting a Nightmare sacrifice detracts 10 seconds. If the timer runs out, the event ends, and starts anew soon after. If enough Nightmares are killed, the tier ends, and players have 30 seconds to find and reach the altars designated for the next tier. As many players as the area can load can participate in the activity.

Tier 1 and 2
No different from the above. 4 and 5 Nightmares respectively must be killed to succeed.

Tier 3
After killing 4 Nightmares, their spawning will be blocked by 2 Major Wizards, floating next to the start point altar and protected by immunity shields. Ascendant Swordbearers will soon exit that altar, and they must be killed to obtain their Cleaver, which can be used to destroy the Wizards' shield. Each Wizard killed adds 5 seconds to the timer, and killing them all will resume the waves of Nightmares. 6 Nightmares must be killed to succeed.

Tier 4
As above, but at the 4 Nightmare threshold, another altar will activate as a starting point, with its own Wizards guarding it. Once the Wizards are killed, Nightmares and Hive will pour out of both altars. 7 Nightmares must be killed to succeed.

Tier 5
The tier will start with 2 starting point altars, and at the 4 Nightmare threshold, a third will activate, with its own Wizards guarding it. 8 Nightmares must be killed to succeed.

Final Tier
The Guardians will have to go to the final altar, in front of the Scarlet Keep's gate. There, the Final Nightmare will appear. Two other Altars, to the right and left, will activate as starting points, and start pouring hordes of Hive, as well as the one further up from the Keep as an end point. Knights in this tier will have Arc Absorption Shields.

After the Final Nightmare has lost a third of its health, it will become immune, and 2 Nightmares will appear from the side altars, soon followed by 2 more from the boss' sides. These will try to sacrifice to the end point, drastically shortening the timer. Once all these Nightmares are dead or sacrificed, the boss will become vulnerable again. This must be repeated again for the last third of its health. Killing the Final Nightmare will end the event, and award the player the daily weapon, and large amounts of Phantasmal Fragments.

Final Nightmares
Final Nightmares rotate every day, in this order:
 * Nightmare of Taniks, the Scarred
 * Nightmare of Zydron, Gate Lord
 * Nightmare of Phogoth, the Untamed

Sacrificial Nightmares

 * Nightmare Knight
 * Nightmare Ogre
 * Nightmare Wizard
 * Swordbearer Nightmare

Other enemies

 * Thrall
 * Cursed Thrall
 * Acolyte
 * Knight
 * Ascendant Swordbearer
 * Wizard
 * Ogre

Rewards
Each weapon is associated with a specific Final Nightmare, and can only be obtained on the day that Nightmare is available.
 * Apostate
 * Heretic
 * Blasphemer
 * Phantasmal Fragments

Trivia

 * This is one of the few activities where Cursed Thralls can be seen running.