What the achievement asks
Kill an elite that is carrying 10 stacks of Vengeance, while the Torment Avenger is running. Ten points.
Avenger is the torment that makes enemies angry about their dead friends: every minion that dies near an elite hands that elite a stack of Vengeance. Your job is to feed one ten stacks and then finish it.
The method
Find an elite with a decent crowd of ordinary enemies around it. Lock the elite down so it cannot join in, kill ten of the small things next to it, then burst the elite before it can make you regret the stacks.
Almost every class has something that works as the lock. Polymorph, Shackle Undead, Freezing Trap, Repentance, Imprison, or plain roots on a melee elite. Hunters can also simply dismiss the pet and use Camouflage.
Do not use area damage while you do this. It breaks the crowd control and, at current gear, it will kill the elite long before it reaches ten stacks.
What trips people up
The stacks are hard to see. The elite collects several debuffs at once and Vengeance is not obvious among them. Count the enemies you kill instead of watching the bar.
Kills have to be close. A minion dying across the room does not feed the elite. Drag them in and kill them on top of it.
The stacks can drop. If you wander too far while gathering the next pull, the buff falls off, which is a bitter thing to discover at nine.
Ordinary elites work. So do the silver rare elites, and so does the run's final boss. The final boss route is slower but reliable: gather the adds, pull the boss, and pick the adds off one at a time near it.
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.
It works again
This achievement stopped awarding in patch 10.2.7 and stayed broken for roughly three months while people filed tickets. Blizzard fixed it in late July 2024. If you are reading forum threads that say it is impossible, check their dates.
Where it fits
This is one of the four under Master of Torment, which is one fifth of Tower Ranger and the Shadowlands meta.