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The Auditors of Reality, who exist beyond space and time, are the Lawful Evil of the Discworld series. Their major trait is that they are very bureaucratic, even more so than Douglas Adams' Vogons, and completely intolerant of change or individuality. However, this makes them very hostile to one of the more recent changes in the Universe – Life.

The Auditors wish to eliminate all life-forms – not just humans, not just intelligent beings, *ALL* life-forms.
As usual, Death said it best:

Down in the deepest kingdoms of the sea, where there is no light, there lives a type of creature with no brain and no eyes and no mouth. It does nothing but live and put forth petals of perfect crimson where none are there to see. It is nothing except a tiny yes in the night. And yet... and yet... it has enemies that bear on it a vicious, unending malice, who wish not only for its tiny life to be over but also that it had never existed. Are you with me so far?

Suffice it to say, these aren't the nicest beings in the multiverse. They even managed to get Death fired from his job in the events of Reaper Man, for having a personality. As a result, he became a human (naming himself Bill Door), and died. Sort of. Almost. He was reprieved just in time, though, and vanquished the replacement Death. The Death of Rats and Death of Fleas remain as a side-effect of this temporary "retirement".

In Hogfather, the Auditors are the start of the whole mess, when they hire the Ankh-Morpork Assassins' Guild to "delete" the Hogfather. However, the plot is foiled, and Death confronts them, and we hear "the wails die away"... but those may not have been all of the Auditors.

Death's quote copied from
Pratchett, Terry. Hogfather. New York: HarperCollins, 2000. pp. 323-324.
A very, very good book, so GO BUY IT!