Quachil Uttaus was created by Clark Ashton Smith in "The Treader of the Dust" (1935). It appears as an impossibly ancient, wizened dwarf of vaguely human shape, embodying entropy and the final dissolution of all matter.
Occult texts warn that summoning it is an act with no return. Contact with the entity causes instantaneous and complete aging and decay, reducing a living person to dust in moments.