Warning: Wikipedia contains spoilers.
The Auditors of Reality, supernatural entities in the fictional Discworld, are celestial bureaucrats: they make sure that gravity works, file the appropriate paperwork for each chemical reaction, and so forth. The Auditors hate life, because it's messy and unpredictable, which makes them fall behind on their paperwork; they much prefer barren balls of rock orbiting stars in neat, easily predictable elliptical paths. They really hate humans, who are much more messy and unpredictable than other living things.
Fortunately for living things, the Auditors can't simply wipe out life, because that's against the Rules; the Auditors can't break the Rules because, in a certain sense, they are the Rules. Unfortunately, a loophole exists in the Rules which allows the Auditors to influence humans into doing what they cannot do directly; in several of the Discworld novels, the Auditors hire humans to perform tasks that will make the world less "messy", paying them with the gold they created out of thin air using their abilities to manipulate reality.
Being personifications of a concept, the Auditors have no fixed shape. When they manifest in the world, however, they almost always appear as empty grey cowled robes, an appearance which conveys drabness and dullness rather than danger.
The Auditors have no discerning characteristics among themselves and function as a collective; when one speaks, it speaks for all of them, and each Auditor works uniformly with countless numbers of other Auditors. In the rare cases when an Auditor appears to develop an individual personality (such as using a personal pronoun to refer to itself or experiencing an emotion) it spontaneously ceases to exist. This happens because, as far as the Auditors are concerned, to have a personality is to be a living being with a beginning and an end, the intervening time between which seems infinitely small to entities who have experienced eternity. This does not seem to have any impact on the rest of the Auditors except maybe as an example to be avoided, because another Auditor immediately takes the place of its vapourised colleague.
Interestingly, the primary opponent of the Auditors' plans for eliminating life is Death. Death does not see himself as the enemy of life, but rather an integral part of it, giving rest to the old and weary, and ensuring that the world doesn't become completely stuffed with life. He has also, over the millennia of performing his function, developed a certain fondness for the humans he ushers into the world beyond.
The Auditors of Reality have appeared in the Discworld novels Reaper Man, Hogfather and Thief of Time.
