I recommend these two things to anyone wanting to attempt Ryuk semi-accurately;
-Shoulder pads, big ones.
-waist cinch or corset
Look into getting pleather or leather pants, pointed toed boots, a belt buckle you can modify to be like his, chains, belt(s), tight long sleeve wide neck black shirt. His wings are tricky because they stretch out so far and the shape isn't common, best bet is to get some commissioned though keep in mind walking around a con with something sticking out that far is a serious pain(at the last con I went to a woman with maybe a 4 foot wing span couldn't even walk around the halls, just stand perfectly still for pictures).
Good luck, this can be quite a tricky costume to attempt.