The Faceroller

The Faceroller is a dungeon & raids achievement earned for killing Eadric the Pure in the Trial of the Champion with his own hammer on Heroic difficulty.

Details
Eadric the Pure is one of the two possible bosses for the second set of encounters in the Trial of the Champion.

Two of his abilities are Hammer of Justice and Hammer of the Righteous (although with much greater effect than player characters have). Eadric will cast the first (stunning the character), then the second (dealing 15,000 damage in one blow).

If the Hammer of Justice effect (a magic effect) is dispelled, the player will catch the hammer and gain the ability to throw it back for a short time, dealing 15,000 damage to Eadric.

Eadric will surrender at about 10,000 health. The strategy, then, is to throw the hammer back when Eadric is at less than 25,000 HP (roughly 3% of health).

Trivia
A common definition of a "Faceroller" is someone who does not require much skill to be successful. The image is of someone who can succeed with as little effort as "rolling their face on the keyboard".