Pops about 15-20 mins after entering zone.South West corner of the Altar Room. Easier than other named mobs in the instance, casts a slow type spell - Putrescent Chains
There are two entries on Illia's for this mob; Putrid Spirit is the right name, but it spawns in the A Corrupted Priest instance, which is where "A Putrid Spirit" is listed.
Finally got him to spawn today, after dragging a bard in with me to check spawn situation. As with many SOD instance names, this guy spawns around 14 minutes after you zone in, no other mobs spawn no matter what you kill or don't kill, so his PH or he is easy to spot on track.
He wasn't at all where I thought he'd be from what little writeups I could find; instead, he's in the big altar room, near the SE corner. Nothing spawns there, but one roamer does occasionally pass thru. And that whole room periodically does an AE dot, Fog of Corruption, this is a 1300 per tic dot which will also blow your invis and potentially your FD. To alleviate this, make sure you kill one of the minibosses for the instance, each drops two clickies, clicking one permanently flags you so you take only minimal DoT damage from this (I was taking 68 a tic)
Anyway, high end hit on me was for 1388, he didn't hit often though so that's not a very good sample. Didn't do anything special, simple mob like the others in the zone, hardest part was finding him.
Spawned 3rd time I got instance after finding his PH (I went thru a lot of instances not looking in the right spot), and dropped the earring first time.
Did the actual task yesterday, as far as I could tell there were no repops or spawns triggered by anything I did, so its probably one of those cases where you get instance, zone in, then wait 15 minutes and see if you got the spawn.
The item I'm interested in, Tarnished Skullhead Earring, isn't listed above as dropping off him, but if you look up the item itself thats what it says.
The post by LaForge is correct with everything as with your own.
Group Mission: A Corrupted Arch Priest Mob: Putrid Spirit Location: SE Corner of the main altar room Drops: Tarnished Skullhead Earring, Tarnished Skullhead Box, and their Type 3 counterparts