Crota's soul. You need it. They have it.
Now it's down to the Moon to fill a crystal with whatever's left of Crota's soul.
You’ll see a cut-scene going through the story from the Dark Below and the rise of Oryx. To reach Oryx, you need to walk in the “dying footsteps” of his son.
You’re heading back to the chamber where the Hive tried to wake Crota’s soul. Just kill the Hive and follow the arrow.
When you reach the arena from the Dark Below mission, Baxx the Gravekeeper, a yellow ogre, appears. He looks nasty, but he ain't so much. Smash it.
When Baxx's health's down, Oryx will take him. Grab the shard of crystal in the centre of the area. It looks as though the mission’s over, but Baxx appears again with Oryx. Leg it.
You have to run out of the chamber and through the corridors, down the the World’s Grave. You'll now be locked into an arena. Kill everything and pick up the floating black relic. Walk up to the door and open it.
Fight your way through the rooms until you come to another locked door. You need to relics to open it. One's on a platform to the right of the entrance. The other is in a chamber underneath the locked door. You’ll find the passage leading to it on the right.
There are three locks for the next door. The first relic you walk past as you enter the chamber; the second is behind a barrier to the right of the door; the third is on a platform in the centre of the room.
Open the door and get up the stairs. Run out into the open and the mission will end.