Spent some time today screwing around with Green Stuff. I've never really used it for anything but gap filling, but in this case it seemed fairly straight forward. Let me know what you think. Beats shelling out $25 for the real thing.
Super simple: Regular Space Marine body, Nurgle Champion head from
CSM sprue, Halberd of Caliban from
DA Command Squad box modified with plasticard and a vehicle spike, a severed Marine head from the vehicle spikybits sprue, and gratuitous green stuff.