The xill first appeared in the first edition in the original Fiend Folio (1981).
The xill appeared in second edition in the Monstrous Compendium Fiend Folio Appendix (1992). The xill appeared for the Planescape setting in the Monstrous Compendium Planescape Compendium III (1998), and the Guide to the Ethereal Plane (1998).
The xill appeared in the third edition Monster Manual (2000), and in the 3.5 revised Monster Manual (2003).
A xill is a 4-to-5-foot-tall (1.2 to 1.5 m) humanoid creature, with gritty red skin, a spiked and horned insectoid head, mandibles, and four arms with clawed hands (not unlike a thri-kreen). Spines protrude from its elbows, and it is both ugly and fiendish.
This evil creature exists only to kill. It lurks in the Ethereal Plane but watches for prey in the Material Plane. It will ambush its victim by teleporting to the Material Plane and attacking with either its four claws, elbow spines, or a variety of weapons (more civilized ones use weapons and hold a different one in each hand). Xill make full use of their Tumble skill in combat: Usually, one or two distract physically powerful enemies by attacking and then assuming a defensive stance while their fellow attackers exploit their numerical advantage. Xill lay eggs in the bodies of defeated opponents, which devour the corpse from the inside out to hatch. Xill also have a paralyzing bite, and using their planewalk ability, can attack through thin air.