What the achievement asks
Kill three special rares in Zereth Mortis: Iska, Outrider of Ruin, High Reaver Damaris and Reanimatrox Marzan. Ten points, account-wide, both factions.
Finishing it awards the Arsenal: Weapons of Glorious Domination, four weapon appearances added to your collection at once.
They are triggered, not timed
This is the whole achievement, and it is why people camp coordinates for hours and see nothing.
These three do not run on a respawn timer like the twenty-nine ordinary rares. They are provoked: killing other rares around the zone appears to bring them out, sometimes within minutes, and there is no schedule to plan around.
When one is coming, the zone announces it. A line appears in chat about a rumbling in the distance and the northern sands beginning to shift. That message is the only reliable signal you will get.
The approach that works is the opposite of camping. Go around the zone killing whatever rares you find, and watch the chat. The chance seems lower on rares already killed that day, so just after the daily reset is the best moment to make the round.
Group finder is the other half
Because the spawns are provoked rather than scheduled, somebody else on another realm has almost certainly triggered one right now.
Watching the group finder for a party on these three is how most people actually get them, and it is far quicker than waiting on your own shard. The names come up constantly while the achievement is being chased.
What they drop
Beyond the achievement's arsenal, each carries its own weapon, and the appearances are learned even on a character that cannot use the weapon type. Nothing appears in your log when that happens, so check the collection rather than the chat.
Where they fit
Dune Dominance sits beside Adventurer of Zereth Mortis, which wants the twenty-nine ordinary rares. The two are the same trip: the rares you kill for one are what provoke the three for the other.