The Dark Within/Walkthrough
From Destinypedia, the Destiny wiki
Ghost tells the player that there's something sealed off within the Skywatch that the Fallen are protecting, and that the Speaker wants Guardians to investigate. Proceed to the Skywatch, the large building at the far end of the Mothyards. The entrance is guarded by a pair of Vandals and a Captain, but they are easily dispatched. Once inside, follow the hallways up to the sealed-off door. At this point, the player will enter a Darkness Zone. Ghost will prompt the player into unlocking the door.
Inside, the next few rooms are completely dark. Ghost will activate a flashlight, but visibility will still be limited. Upon entering the larger room, approach with caution, as a group of Thrall will attempt to ambush the player, either by charging ahead from the shadows, or jumping down from the ceiling. A group of Acolytes will also start shooting; at higher difficulty levels there may also be a Knight. Dispatch the Thrall first; retreating into the doorway will funnel the thralls into a single file, making them easier to manage. Once the room has been cleared, proceed upstairs.
Ghost will warn the player that a Wizard (Gotra, Eir Spawn) is nearby. Before the Wizard can be dealt with, more Thrall and Acolytes appear. The Wizard is inside the final room, supported by more Thralls and Acolytes; it is not completely necessary to eliminate them all, since killing the Wizard will kill all the other enemies.
Before the mission ends, there is a Golden Chest on the roof above the enclosed room.