What the achievement asks
Complete all four cypher trees of Zereth Mortis: Metrial, Aealic, Dealic and Trebalim. Ten points, account-wide, both factions.
Finishing it unlocks the Pale Regal Cervid directly.
Every rank, not just the first
This is the thing to know before you plan anything, because it changed after launch and older advice still says otherwise.
When the achievement arrived, one point in each cypher was enough. A patch in January 2022 changed it: the achievement now requires every rank of every cypher researched, and a tree sitting at rank 1 counts for nothing.
The difference is not small. Rank 1 research runs in hours; rank 2 takes closer to two days, and rank 3 longer still. The achievement is measured in weeks of waiting, not in cyphers gathered.
Keep something researching at all times, including while you are away from the zone. An idle console is the only thing that genuinely costs you here, and cypher research runs whether you log in or not.
The mount no longer needs a schematic
Older guides mention a Schematic: Pale Regal Cervid dropping somewhere. It does not any more: since patch 9.2.0 the mount is granted directly when the achievement completes, and the schematic became unobtainable.
If you are hunting that schematic, stop. Finishing the four trees is the whole route.
What else the research unlocks
Cypher research is not only for this achievement. It is what powers Pocopoc, and three of the treasures in Treasures of Zereth Mortis cannot be opened without Dealic Understanding and Sopranian Understanding, each a week of research on its own.
Since both achievements draw on the same console, plan them together: the trees you complete here are the trees those treasures are waiting on.