Repentant Angels actually look a lot like how I picture Odio, and might not really need much reskinning. I haven't looked at the model lately (and don't have time to now, as I'm up trying finish my Structural Analysis homework and only online to break the monotony to try to stay awake), but the only thing that really comes to mind is that the little bit of white cloth should probably be a different color. You might want more changes too though, so it is easier to tell the units apart without having to make him so much bigger that he makes nearby units hard to select. I could see Odio having a less natural skin town, maybe a dark shade of purple or something close to the gray color he looks in statue form.
Currently in my version I have a spell called Unleash Odio that any unit can cast on the tile once the owner knows Rage. I used to have this kill the caster and summon Odio in XML, but I decided against letting Odio get the summon perks of the unit that freed him and I found myself always sending a cheap expendable unit to summon him and figured I might as well get rid of that too. Since python was needed to remove Odio's Prison (and the body mana in contains in my version, and FF too I believe), I figured I might as well create Odio there too. I gave it a really high AI weight, and so I find that an AI almost always gets Odio before I can.
My Odio has Demon, Hero, Crazed, Fear, Aeron's Chosen, March, Water Walking, Blitz, Body 2, and Chaos 2. He also gains Enraged from combat (with living units), so while it is pretty easy to get him he is very hard to actually control. He has 2 moves, strength 6/1 + 4 Unholy + 2 Poison + 2 Body affinity. (My Brigit is stronger than normal too.)
I also created another spell, Bury Hatred, that does the opposite of Unleash Odio; if a Runekeeper can get within range of Odio, he can use the power of Kilmorph to draw the demon back into the earth, destroying the unit and creating a new Odio's prison on its tile.