You could tackle it in a range of ways - Im also studying at uni so im by no means the most informed artist but I would look into either…
a) Model the reaper like a normal mesh and then use it as a container for particles (could be very time consuming going by my own tinkering)
b) Again, model the entire thing, and then give it the ghostly look in AfterEffects/nuke/your chosen post-production software.
Going by discussions with my tutors it is possible to animate a normal map, so perhaps if you were able to rustle up a cloudy texture, make a normal map of it, and animate it that way. I say this because i imagine option (A) would not give you the definition you’d want in the characters form, and a normal map would interact with the lighting and give the necessary shadows.
Though i do want to make it clear these are things which i’ve only considered doing theoretically, would help if someone could validate whether any are possible/worth pursuing.