A yochlol is a type of lesser demon who act as "handmaidens" to Lolth, the drow goddess of chaos. Their natural home is the Demonweb Pits. Only the Queen of Spider's high priestesses are told how to summon a yochlol and they are only supposed to be called upon in dire need or to witness sacrifices in the name of their dark goddess.
In their natural form they appear as tentacled blobs of melted wax and continually emit a foul stench. They also have the power to change their shapes—usually to a comely drow female (although other elves and humans are possible for them) or to a large spider (usually a black widow). They can also change themselves into a gaseous form, allowing their unknowing victims to inhale them, and then suffocating them.
Lloth created the Yochlol from succubi she captured from Abyssal layers. She has never revealed what she does to transform the succubi into yochlols but when the process is complete, the yochlol remembers nothing of it's former life and is completely loyal to her new mistress.
Yochlols are chaotic evil creatures, as are all tanar'ri. They take great pleasure in dominating other, lesser creatures. They are cruel and enjoy a good battle, exulting in the rage that fills them whenever beings defy their goddess's will. All yochlol serve Lolth and work surprisingly well together. Never will one betray, endanger, or attack another. All yochlol share a telepathic bond. They take special delight in escaping the control of those who have summoned them (whom they must serve faithfully for one deed, as decreed by Lolth), and wandering free about the Prime Material Plane. Such a yochlol uses its various forms as disguises to carry out cunning ploys and evil subterfuges to turn beings to chaos and evil. Unless enemies of Lolth are present, yochlol will not engage in killing sprees or frenzies of mindless violence. Yochlol often hiss, whisper, or scream when in combat, even telling their opponents their names, in case the victim should escape. In that way, the victim knows that a particular Handmaiden somewhere out there will someday return to finish its business with the character.