Infiltrators

These rares show up as the same model as [npc=152708], name included, except they have the silver dragon rare icon on their nameplates. Once you engage them, they reveal themselves as the three rares which are the target for this achievement.

So far I've found [npc=179136] and [npc=179133] farming layer 9 Skoldus Halls. I haven't yet run into [npc=179135].

I found all three of these rares while spamming Layer 6 Skoldus Halls runs. They appear as normal enemies but have silver dragons on their portraits and appear in the spots where rares normally do on a floor template. Once attacked, they turn into demons that cast powerful spells (Shadow Bolt Volley, Carrion Swarm) you should interrupt.

To farm this achievement efficiently, go into layer 3 runs of your chosen wing and check for rares in floors 1 and 2 before leaving if you don't need the soul ash at the end.

Found Sath'zuul on first layer of Fracture Chambers adn Karthazel on L10 of Mort'regal.