What the achievement asks
Kill the run's final boss without ever killing or interrupting the Mawsworn Doubtcaller, while the Torment Backup is running. Ten points.
Backup gives the last boss a helper. The helper is annoying, it slows you and puts damage over time on you, and everything in you wants to deal with it. Do not.
The boss is the last boss
Not a floor boss. The final boss of the run, at the top. People spend an evening carefully sparing the Doubtcaller on floor 3 and wonder why nothing happens.
What counts as interrupting
Far more than you would think, and this is where the achievement is actually lost.
Any crowd control counts. Fear, Hex, Polymorph, Freezing Trap, stuns, knockbacks. Several people have failed it by sheeping the add before the fight even started.
Passive and automatic effects count too. Anything on your bars that carries an incidental interrupt, and pet abilities that do the same, will end the attempt without you noticing.
Anima Powers count. Take a moment at each Hoard: anything that stuns, silences or knocks back is a trap for this run specifically.
The safest version is the dullest. Single-target the boss, use no area damage, no crowd control, and no cooldown that does anything clever. Let the add hit you.
Where to stand
The Doubtcaller tends to stay put if you do not touch it. Pull the boss twenty yards or so away and fight it there, and your cleave stops being a risk.
If your class can drop threat or break line of sight, that works too: several people have finished this by tagging the boss and dragging it somewhere the add simply never engages.
One disputed point
A well-upvoted report says you must land at least one hit on the Doubtcaller yourself, and that pet damage does not count, or the achievement will not register. Others report ignoring the add completely and getting it anyway.
At current levels one hit from you is likely to kill the add outright, which fails the achievement definitively. Ignoring it is the safer read.
The rotation is the real obstacle
Blessings and Torments are fixed per wing and reroll when the daily quests do. You cannot pick the one you want: you check what is up, and if it is not there, you come back tomorrow.
They are not the same across the six wings on a given day, so check all of them before giving up. People find the torment they need sitting in a wing they never usually run.
Layers matter too. A run carries more torments the higher the layer, so if the one you want sits late in the list you have to go deep enough for it to be active at all. Roughly: layer 8 for one, layer 10 for two, layer 12 for three, and the last slots only appear at 15 and 16.
Where it fits
This is one of the four under Master of Torment, which is one fifth of Tower Ranger and the Shadowlands meta.