Well i dont want to spell anything out, but perhaps get you thinking on what you might want to do.
Well i think he should have a wildshape form, if nothing else just to let the DM and the players know that you are wildshaped. But you could say your wildshape looks undead and frankenstein's Monster-like. I mean you see a patchwork man tearing at your allies is one thing, but a patchwork Crocodile? i would run! And if you really want arms just go gorrila :P (though if you realy wanted to, with dm's go ahead, say he stays humanoid, just add a few features, like perhaps shapened teeth, claws, elongated arms, bone plates/spikes, and such)
Now as for backstory he could have been a typical Shadar-kai, just born small and picked on. One thing i would try to focus on is his death and resurection. Is he goood aligned and try to help someone else and got killed for it. Was he a theif or murderer and was executed. Perhaps he was mugged or jumped by wild animals. Or did one day he just fall and break his neck?
Now as for bringing him back to life was it the raven queen herself? Did a necromancer get ahold of his body. Perhaps a wizard was trying to create a Flesh Golem and carved a rune into your head (like the classic golem) and somehow brought you back as a Revenant? Perhaps he does not know why he is back or why he died for that matter.
How devoted to the raven queen is he? Does he pray every morning and night, or does he only pray when he feels like it. does he wear his faith like a badge or does he hide it from the world. If he feels his god is slighted will he be ok with it or will he get aggresive.
Perhaps you want to play up his primitive side. He has both primal and elemental like magic. Does he feel a close connection with nature? Would he side with a water elemental defending its home? Did he have those connections when alive or just after he died?
As a runepreist does he know what the symbol means? Did it show up after coming back to life? was it carved by the raven queen like a brand? Did it come to him in a dream and he carved it on himself? Is he curious or dissmissive to it's power.
How does he feel about the living and the dead. can he still connect with peaple or even his own race? are peaple just tools or even just "food" for him. Has he meet other like his kind? Are undead abominations or are they kindred souls? What body parts whould he attach? Would he use animal flesh? What about he killed a child, would he then attach it? What about if he had to kill another follower of the raven queen? Does he try to hide what he does with his enemies or does he do it openly in front of his allies, no matter what they might think.
Does he like what he has become (since he race idolized the raven queen and the undead). Is he happier now then he was alive, or does he want to perhaps live again. IS there anything he misses, either simple things like the taste of food, or complex things like freinds and family. How did his community take his return? was it celebrated, or was he cast out.
Finally why does he asventure? is he mearly looking for kills to add to his flesh? why not become a murderer then? Is he looking for something or someone? A holy quest by the raven queen or tying to track down the person who killed or raised him? Is he just lonely and want companions?
Anyways this is just some stuff that might help, good luck
