(We be fillin' the nodeshell . . . )
Yuch-lull (With a throaty "ch" as in "Bach")
In AD&D, Yochlol are a form of lesser tanar'ri (a.k.a. demon). Specifically, they are the direct agents of the drow goddess Lolth, and are sometimes referred to as Lolth's "handmaidens." The Yochlol, as a whole, have a closer relationship to this Power than other tanar'ri do to any other Powers. Unlike most other tanar'ri, who have been embroiled in the Blood War with the Baatezu (a.k.a. devils) since shortly after the dawn of time, Yochlol exist only to serve the Queen of the Demonweb Pits.
The natural form of a Yochlol is a 6' amorphous heap of slime with eight tenticles and a glaring red eye (a form it usually wears while on its home layer of the Abyss with Lolth), however they may also take on the form of a human or elven female (a form they often use for travelling on the Prime Material Plane), or a giant black spider.
Although some might claim that Yochlol are not actually tanar'ri, since their lives tend to be pretty removed from "normal" tanar'ri society, they've got it all wrong. Fact is, Yochlol are recruited from the ranks of other lesser tanar'ri, subjected to a nasty "evolution" process, and then bound to Lolth. They might be quiet by tanar'ri standards, but they're still in the family.