I let the wraith and witchunter rest a while and instead quickly painted up a conversion that had been standing on my desk. Nothing fancy - body from a chaos cultist, arms from cadian command squad and a flagellant head. But then he's just a cannonfodder for my GK-army.
Two things to be learned from this model:
1. The chaos cultists are a suberb base for conversions.
2. Plasma pistols are, even by 40k standards, ridiculously oversized.