Spawns on the shade island just south of the elemental island. I left a char parked there for ~10 hours and I saw 1 Ranvin. This was also during increased rare week.
All I have to say about Ravin is you better be prepared for a bad fight even at level 55(my pally)...he'll life tap your as* off as well as fights like a monk/SK and even though he looks human, Instrument of nife(undead wepon proc) will hit him. I dont know what to call this guy short of one badas* ****. So if you plan on trying to get the ring or idol from him, be ready to die. I've watched a 58 pally die at my 56 Shamas feet when I was oom and could do a thing but sick my pet on Ravin and head for the hills. He seems to be a bit of a rare spawn there but all you have to do is sit tight and kill the other named shadows as you find them...he'll be along sooner or later.