Lol I was thinin' about this the other day, not sure if it would work at all since I haven't seen either models
irl...
Anyways the idea was to cut the body off of the nightbringer model & replace it with the top half of this model
Cut off the necklace, replace the club with a giant scythe of something along with green stuff to blend to models in together appropriatley, most of the green stuff could be used just to replicate a similar fabric that's wrapping around the original c'tan models which would cover up the transition quite nicely.