I originally picked up a Cryx helljack to make Malice from, but I got a better idea. This left me with a spare helljack, so I decided to convert a plastic Nightmare. Not that there is anything wrong with the stock model (I have plans for that mini) but it wasn't scary-ghost-robot enough.
A quick dig through the bitz boxes unearthed some large dark eldar blades, talos tails, daemon horns, and the spikes from the Anima mini I made into a warwitch. Since I envisioned him as a mini wraith engine, I did away with the legs, using the helldriver as a tail. The dark eldar blades replaced his claws, as he has "wicked claws" with reach.
Like most of my minis, he got a fancy base with a nice chunk of cork to stick the pin through. With five pins total, he is a pretty stable mini. The metal spikes are glued into holes drilled into the shoulders, instead of trying to make a hole in the spikes, which worked extremely well.