“Oryx has taken notice of you and sent an Echo of himself to kill you.”
Just after the Guardian lands at the top of the lift, you will see a number of Fallen. just after that, a Shadow of Oryx will teleport in, killing all the Fallen in the room. This Shadow is a dangerous opponent. It will teleport from the center off to one of the sides. After you have dealt some damage to it, it will teleport away.
Just before it reappears, it will summon in a number of Taken Psions. Quickly eliminate them to avoid getting swarmed by them. The Shadow will not reappear until most of the Psions have been cleared out. When it does, get some quick hits in. Before long, you will see a notice that "the air around you crackles and burns." Immediately move from your location to the next section of the arena. The Shadow is loosing a powerful area effect. There is a circle that appears to denote the effected area. Get a line of sight once you are safe and start shooting the Shadow.
When you get the Shadow close to death, it will start calling in more Taken Psions and some Vandals. This time, it will start flying around instead of teleporting. Be on guard for its area attack and Void Bolts. As soon as you defeat the Shadow, the Taken will vanish from the area.