For the torso, use the tactical marine chest that's plain except for the single skull in the center. GS the wings.
Legs - the older mark that comes with the assault marines (the one without kneepads). Cut it at the hips, and possibly knees, reposition and GS the gaps. GS straps across the thighs, and sculpt the tabard yourself to fall between the legs. Sculpt a face onto one of the greaves.
I'd use the claws that come with the chaos terminator lord. Chop them at the wrist/forearm and attach to power armor arms. I'd use bolt-pistol wielding arms (or chainsword) as they are not molded to go across the body, but rather go straight or to the sides, so he can appears to be swinging them as he walks.
For the head, there may be a SW head that's suitable, but I'd be more inclined to go with an eldar or high elf head, possibly gs'ing more hair onto it. It'll look more gaunt than a marine head with gs hair, and give him a more haunting look, I think.
For his pack I would use a regular SM pack, cut off the exhaust vents, and replace with a pair of the wings cut off of the night lord helmets.
His shoulder pads could easily be gs'ed, I think that would probably be the best choice, unless you have some of the spikes and whatnot from various CSM sprues.
edit: Just notice the rim around the shoulder pads. Use the inquisitor/deathwatch shoulder pads from the SM captain, shave off all the detail and add the spiekes