What the achievement asks
Clear Layer 4 of The Jailer's Gauntlet. Ten points, account-wide, and the Sturdy Soulsteel Mawrat Harness, which teaches the Colossal Ebonclaw Mawrat.
There are eight layers and they open in order, so this is four runs rather than one.
It used to be layer 8
Tower Ranger originally demanded Layer 8 here. Patch 9.2.5 cut it to Layer 4, which is why older guides and older complaints do not match what the achievement asks for now.
That matters, because the Gauntlet's difficulty curve was famously not linear. Layers 1 to 3 were a formality, layer 4 was where people stopped, and 5 upward was where they stayed stopped. You are being asked for the last easy one.
What it is like today
At current levels, nothing here holds up. The Shadowlands-era advice about hunting for specific Anima Powers, chaining Aspect of the Turtle, or bringing a group to whittle the boss down describes a fight that no longer happens.
Run it, kill things, take whatever powers appear.
If the achievement does not fire
This one has a long history of not awarding on completion, and of awarding to one member of a party and not the other. It has been reported that way from 9.2 through to Dragonflight.
If you finish layer 4 and nothing pops, run layer 4 again. Multiple people report getting it on the second run with no change in what they did. It is not something you did wrong.
The mount is not an item
The reward line names a harness, but nothing lands in your bags. The Colossal Ebonclaw Mawrat goes straight into your mount collection. It is also, despite the name, one of the smaller mounts in the game.