I lack three rares to complete this achievement, but this NPC is a mystery; it appears he was never found even on the PTR.
What is Known
1) He is a "seer" type of demon.
2) He has an "unstealth" sound effect attached to him, per Wowhead.
3) So far, he has not appeared as a WQ, random spawn, and I have never seen any groups for him listed, and some have reported that he was never encountered on the PTR.
Points to Ponder
1) His name would indicate his job is to attend to "souls". There are a few places that might fit the bill, specially in Northern Krokuun and Antoran Wastes.
2) Since many seer demons are found in Antoran Wastes, that may be the best bet for his location, despite there being quite a few rares already in the zone.
3) He may require an "attunement" akin to The Many-Faced Devourer or Squadron Commander Vishax. I've seen no strange item drops in my time in Argus that would indicate this in the case, however.
4) He may not unlock until after quests in Week 3 are completed.
5) He may become available to kill when all the other rares have been killed, a la Xemirkol in Tanaan Jungle. However, Videx is included in the achievement list, whereas Xemirkol was not.
6) He is not yet in the game, or he was mistakenly left in the achievement.
Things to Try
1) Kill only the seer rares in Antoran Wastes (maybe after the other rares have been killed?).
2) Explore the entirety of Antoran Wastes while under the "agent of the all-seer" buff.
3) Try out the Vindicaar powers in out-of-the-way areas.
4) Explore Argus while in ghost form.