“Drop down the elevator shaft.”
“Descend to the Iron Tomb” is an objective in the mission The Iron Tomb in Destiny: Rise of Iron
Walkthrough[edit]
The room below the one you were just in has a lot of Splicers for you to deal with. There are Dregs, Vandals and Tracer Shanks. CLaer out some of them and turn to the right as you enter this large room. There are some SIVA tendrils in the back of the room. Below them, in the back center, you will find the entrance you are looking for to the SIVA complex itself.
Just go up to the edge and drop off. You will survive. A well-timed activation of the secondary jump never hurts though.