This particular rare mob spawns in Iron Pine Peak, within seconds of it spawning it rushes into a nearby foothold and dies, if for some reason the foothold isnt there (it always is, since Iron Pine Peak isn't the busiest of places). The rare mob will try and run across the bridge, which is where the invasion forces running over the bridge gets him.
Netherworld Wanderer is probably the easiest rare mob in Iron Pine to find. I have killed him several times across my characters, and even watched him run to his death several more times for amusement.
I have only seen him once, and that time he was dead. I generally check 2- 3 times a day, for at least the last 8 months. I have checked at reset, 3 hours after, but he is never around. I have to admit killing all those footholds helped me towards the achieves, lol. I have the same issue Hogger. Someday...someday