That's a fantastic conversion. I particularly like the way that you have posed the model to be pointing menacingly towards his target with that maul.
As for naming, as Dark Apostles were, at least originally, exclusive to the Word Bearers, I'm not sure that I'd make his name favour any particularly Chaos God. [Insert name] Chosen of Lorgar might work, but it's rather unoriginal. How about some alliteration? Drazhar, Demagogue of Darkness might work, for example.