I'm not even sure what I'm going to use him as, but I love how the model turned out so I'll find a way to fit him in somewhere. I thinks he's got a distinct Cthonian or White Scars look about him, so I'm thinking of probably painting him up as a traitor member of the fifth legion who's now serving my Sons of Horus, maybe having him fluffed-out as being one of the rebel 'Scars who fled after the battle above Prospero.
Looks like I'm saving my Mal conversion for another day, once I can get hold of the Huron parts again. Oh well, at least I got a cool homebrew character out of it.