That looks really good, but I think the pylons should be pointed inward a little more at a pivot point at their base, maybe. Like the Ark Portal on Earth. Other than that, fantastic job.
As far as the Onyx Sentinels go, I seem to envision them with a vaguely Monitor-esque central sphere. Not directly like a Monitor's though, but close since these sentinels had a higher grade intelligence than those found on the Halo rings or anywhere else it seems.
Maybe pull back the outershell around the eye a little bit and duplicate that smaller downward protrusion twice more, turning it into a triangular design around the eye. And remove the larger downward protrusion and make the rest of the exterior more symmetrical, possibly even fully sphereical. And remove the propulsion thingy from his back and round out that out.