A new event has arrived on the Moon in Destiny 2's most recent update. The Altar of Sorrows is practically the lunar version of the Escalation Protocol. To access it, you will need to visit Eris Morn and finish the Shadowkeep storyline. It will have a mission waiting for you and you can then fly to Sorrow's Harbor to complete it.
Destiny 2 – Altar of Sorrows Guide
The starting point for the Altar of Sorrows will be marked on your Director. You should be able to see large green orbs of energy on all altars in the area. The event boils down to destroying all the nightmares that come out of the spheres. As each level progresses, the number of nightmares you have to kill increases.
Towards the end, the Mages will start spawning protected by shields and you will need to kill the sword-wielding Knights to get a shield-breaking weapon. You also have to kill the Nightmares pretty quickly, as you have to stop them from walking from one sphere to another. It sounds a little more complex than it is, but it mostly comes down to killing everything.
For the mission you are trying to complete, you only need to kill 20 nightmares and reach level 3, but the event itself will progress to level 5 and culminate in a final boss fight on the central altar near the Shadowkeep itself. I assume these will be bosses in rotation, and this week is Phogoth's nightmare.
The usual Nightmare boss fight rules apply, from time to time it will become invulnerable, spawn into other Nightmares and you will have to kill them before they reach the orbs to be able to damage Phogoth again. If you manage to get it out, you'll get some nice loot, and it should be noted that this event is releasing new event specific loot.
When you're done, go back to Eris and deliver your mission, and now you'll have access to the new Pit of Heresy dungeon! Dungeons are big strikes or small forays, depending on your topic. You'll need three games to make it, just like the Shattered Throne. The Pit of Heresy will appear on your map in the Director, so head back to Sorrow's Harbor to reach it.
