I didn't get it checked off, but it was a Sand Dweller.
Sand Dwellers dwell in the Desolate Sands. ;-)
I think the problem was that I was using my hero that I created on launch night back in May. So, just like I had to use an alternate character to get all the lore books, they wouldn't drop for him because he had already found them but the achievements had been reset, I'll have to use an alt to hunt down the Sand Dweller because the game has already registered a rare Sand Dweller kill for my DH.
According to DiabloWiki Champion/Rare Sand Dweller can spawn in the Unknown Depths. I have done many runs there and found out that it can spawn only as a guard when there is a resplendent chest. But it always spawned as a champ (it was about 2 times from about 50 runs when there was a resplendent chest, so maybe it is possible to spawn also as a rare pack).
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