The observers — rare, reclusive, psionic beholder-kin — are, unlike their brethren, quiet and withdrawn when roaming the planes, a surprise to many the Prime that's come across them in their travels. Far from harmless, though, they're a danger to any that happen across their own small "empires" uninvited; it's few the soul that have entered an observer's realm and come out unscathed. The realms of these creatures can be found dotting the secluded areas of the Law-side Outlands, or the various planes of Law but for Mount Celestia; even in Arcadia can be found the occasional observer.
The specific details of the origins of the observers are unknown. All that can be said for certain is that they descend from planar beholders some time in the distant past, having slowly drifted from that species over the eons in both appearance and nature, gaining psionic powers along the way. Beholders seem to have a high degree of ire for observers, but they have an ire for all beholder-kin, so little can be read into this.
Observers are solitary creatures, most often encountered during their wanderings of the planes in search of new bits of knowledge. Outside of their empires, observers are passive, unobtrusive figures, preferring to watch events from a distance without interfering to better gather more information. When encountered in the field, they tend to be quiet, withdrawing from others unless they happen to have something the observer wants; while at times they may attack for it, more often the observer will simply negotiate for the desired bit of knowledge or item. On occasion, they will even agree to trade one of their own holdings of knowledge or arcane minutiae, leading many to seek them out in the field specifically to offer such an exchange.
Things are very different in the territories claimed by a given observer, of course. Eager to remove all unpredictability and irregularity from the areas they claim for their personal "empires", observers apply their psionic powers to take control of all intelligent beings within their realm, turning them into slaves applied to keep their realm in perfect order; not out of any malice towards other beings, unlike most beholders, but rather simply out of a sense of paranoia and an obsession towards things functioning "properly". Any outsiders that happen to enter the ill-defined bounds of their realms are at risk of falling under the same sway, but thankfully, the strange behavior of its occupants — stilted, regular movements among the people, a lack of native life, and perfectly-kept environs even out-of-doors — often is enough to drive them off before being taken themselves.
These beholder-kin have much in common with true beholders; much like them, they are pure carnivores, able to feed on near any sort of meat to sustain themselves. Their abilities tend to make hunting near-trivial, and so they rarely have any trouble finding food either at home or during their wanderings. Their lifespans are unknown, given their reclusion and hesitance to speak on themselves. Their anatomy, however, is far-varied from their ancestral species. They've grown to a tripartite anatomical structure, with three evenly-spaced eyes and mouths about their spherical forms, the mouths mounted on retractable stalks reaching up to five feet from the body. Further, they've also lost the internal air bladders that grant a beholder buoyancy and flight, instead having gained an innate levitation ability that keeps them aloft.
They reproduce rarely, usually once or twice over the course of their life, and in a matter vaguely similar to the slaadi. Hermaphroditic beings, when the urge takes them they implant a parasitic egg into the body of another being; most commonly one of their slaves, but any creature close to the same size as a humanoid will do. After three months, the egg hatches, consuming its host from the inside before emerging at about a foot or two in width. The newborn observer is quickly driven from its parent's territory, and grows to full size and power over the next two years before founding their own empire as far from their parent as they can manage.
Observers at full size are nearly-spherical beings of about six or seven feet in diameter, with a mottled purple and pink, chitinous exoskeleton up to three inches thick at parts. Their bodies hold three large eyeballs about the size of a humanoid's head, evenly spaced about the equator of their bodies, each with two small nostrils and a wide fanged mouth below it. Like a beholder, they also have a set of eye stalks growing from the top of their head, though more spindly, more akin to a snail's. Thin, stiff hairs cover their exoskeleton, twitching lightly in the air, but to no apparent purpose.
- Planescape Monstrous Compendium Appendix II, pp.70-71