I just figured out how I would go about it. Icon Venom is the closest you're going to get proportion wise. Now your looking at probably Mcfarlane Urizen for the head. You'd need to extend the toso by cutting and glueing in some plastic, maybe from Urizen, using a saw and plastic epoxy. Doing the same with the forearms and thighs. Probably either cutting the feet or removing the and adding a second set of lower legs(similar to ML Blackheart). Hot glue for the flames and lots of sculpt to cover up the cuts and added plastic. McFarlane Dragon wings would work well as most are fairly large. The only figures in the scale you are going for are ridiculiusly expensive and do not have the proportions for Balrog. This is a large undertaking IMHO however you go about it. I'm sure there are less articulated ways of doing this as well. Personally I'd even cut the Venom between the hip joint and the torso swivel. Cap them off by glueing some round flat pieces to both ends and then glue the plastic for the extension there. Again, just my preference and how I'd do it. Might even try using Dark Alliance Lucifers horns and glueing them to the head for a reference on the build up.