Nastyish mob for its level, not to be taken lightly. The snare has landed every time on my puller and a 90% snare means he ain't getting back to the full group any time soon. Best fought at his spawn point or close to it and be sure to have cures and/or group heals available.
I also noticed he doesn't seem to assist ordinary oashim, or his agro range is tiny, as I was able to pull nearby rats without this named aggroing.
The hunting around that abandonded house is correct.
There is a small flat area just north of the friendly Rallos mobs that is nice for pulling to for this camp. Watch out for the one roaming spider that paths thru there.
I had named pop from each of the static mobs on the outside of the house - 2 on the back of house, 1 on each side, saw him pop from all of those as well as the roamer mob that pops in and out of house and then paths away a bit. I didnt see him pop from any other roamers apart from that one.
Spent 15ish hours farming the sword. He popped a total of 5 times, dropped waist item first three times, then I was about to give up and he dropped sword, popped back to back and dropped sword again ... /boggle.
Seems to spawn off of Oashim with any kind of prefix. Like dirty, mature, etc. As you pass Rallos Zek's camp, there is a house that sits there with a bunch of rats. I found him there. He popped from a mature oashim. Killed with a 95SK and merc. Fight took about 10 minutes. Lots of hps and the dot is manageable. Dropped Vermin Chewed Sash!
PHs are any oashims near the loc given (around the house). Kepp the area clean, for a safe pull. Straight forward fight. Cast the dot 3 times during fight. Be careful, because targets of the dots are generally clerics and slowers (as usual...).
This named (Oashim Progenitor) spawned at in-game /loc 1130, -900, -49 . This is along the east wall of an abandoned house, northernmost of two static rats that spawn there.
For your arelis_3.txt map file:
P 900, -1130, -49, 127, 0, 0, 2, Oashim_Progenitor
Data please: PH, where in Lunanyn does he pop. Group size needed to take him down. How many times did he pop over the course of 1, 2 or 3 hours. Stuff like that.