From Destinypedia, the Destiny wiki
The player presses deeper into the Enduring Abyss, eliminating any Hidden Swarm trooper standing in these dark catacombs. After making it into the opening revealing Earth, the Ghost will begin to report being invaded by a mysterious force again.
The player meets face to face with the dormant Pyramid once again.
After pressing through their final Hive opposition of the mission, they finally circumvent the Nightmare barrier keeping them away from the Pyramid. They continue down the cliffside towards the Pyramid.
Scaling further down the cliffside, the player finds themselves at a dead-end with only one platform left. Descending down, however, the player is then seized by the Pyramid and is drawn into one of it's open hatches.
The player is pulled into the Pyramid, into a new location known as Admittance.
The player then explores the Pyramid.
The Nightmare does the same thing the real Ghaul would, except uses the immunity ability all Nightmares possess. Before the player is able to defeat him, the Nightmare heads to the pedestal where it's template would have gotten his power and the Guardian is teleported back to the Pyramid.
The Guardian continues to explore the Pyramid, eventually finding the Nightmare of the Fanatic.
The player fights the Nightmare, but again is teleported back to the Pyramid before they can defeat him.
The player then explores the Pyramid more still and encounters the final Nightmare, the Nightmare of Crota, Son of Oryx.
The player defeats and takes the blades of Swordbearer Nightmares, and defeats the Nightmare of Crota.
The Guardian climbs a flight of stairs, leading to the apex of the Pyramid. There, they find a sculpture, cast under a veil. In front lies an Unknown Artifact.
The Guardian inspects the Unknown Artifact, but are then teleported into a vision.
The player turns behind them and sees...themself. They realize that the Darkness has assumed the form of them.