Rarely storied, and almost never encountered, the Beholder is a creature that strikes fear and awe into the hearts of any who witness its alien shape. Their origins are unclear, and the Beholders themselves have no interest in their own genesis.
As a result, the source of a certain creature so different, and yet queerly similar to the Beholders is even more arcane. This creature, the Eyeleech, has a completely different mode of existence than the Beholder, and exists in almost an animal state; and rather than preferring the utter, self-enforced solitude characteristic of almost all Beholderkin, the Eyeleech depends on another creature for its defense and transportation, and hence, its very survival.
The Eyeleech has a form similar to the Beholder, but in a much more diminutive aspect--most are no larger than a fat and horrendously repulsive muskrat. Skin, floating ability, and general shape are all like those of their abhorrent kin, although the Eyeleech is considerably less able to maneuver during flight. However, the central eye common to the true Beholder is non-existent, its space taken up with a relatively enlarged mouth, and quite unlike its cousins, the Eyeleech's much reduced size is paired with a very limited ability to reason independently--in its normal state, the Eyeleech is barely more self-aware than a smart dog or horse.
Each Eyeleech possesses eight eyestalks, of which four are longer and more flexible, and do not actually terminate in eyes, but in toothy, mouth-like openings. These "latching" arms serve a key purpose for the Eyeleech, and it is through their industry that the it earns its repulsive name, for they are used to ensnare and attach to unsuspecting or helpless creatures and incorporate themselves into the creature's body, dominating its impulses for the Eyeleech's benefit. It is here that an Eyeleech's preferred mode of existence comes into play, for it is by intruding upon the host creature's mental structure that the Eyeleech obtains reasoning capability and augmented physical prowess. When attached to other dumb beasts, this results in a more vicious and dangerous encounter. It is thought that an Eyeleech attached to a creature capable of higher thought would be both monstrous and deadly.
Being both smaller and less intelligent than their true Beholder cousins, "unlatched" Eyeleeches (that is, Eyeleeches without a host) are stealthy and timid, staying out of site and sustaining themselves by ambushing smaller creatures for food. They use little magical energy in this phase, since they work solely from reserves that they have either been born with, or that have been absorbed from their last "latched" phase--they rarely use the capabilities of their magical eyes except as a reflex in an emergency, and do not possess any particular tactical prowess to speak of.
Every once in a while, however, the Eyeleech will find a larger creature alone and unaware, and all timidity will recede as the Eyeleech risks latching itself onto the host, which it realizes will provide it with many more opportunities to sustain itself. In these ambushes, there is no imperative to conserve magical power, and the Eyeleech will make use of any immobilizing ability it has (including a venom secreted from its latching stalks, unique to the species) to subdue the potential host.
The latching process can take some time, and during this period, the Eyeleech will endeavor to keep its host immobilized and break down its will. This is accomplished by first attaching itself to the victim as close to its brainstem as possible. The latching arms will then simultaneously inject the victim with a paralyzing agent, and extend feelers beneath its skin to make contact with the brain. Once contact is established, the Eyeleech will bear down upon the victim's consciousness, breaking down the creature's sanity until it snaps, allowing the Eyeleech complete control. In this phase, the Eyeleech is very vulnerable to outside attack--if it perceives itself in danger during the process, it will make one last attempt to seize control, and if that is not successful, it will forcibly retract itself and flee as quickly as possible, generally killing the victim in the process (or leaving it in a gibbering, useless heap).
Once the Eyeleech has sufficiently dominated its host, its consciousness will make use of that creature's mental capacity. What thoughts remain in the host's mind will blend with the Eyeleech and subtly influence its psychology--with particularly strong-willed creatures, this can even manifest as a hybridization of the Eyeleech's mind with the host's, creating a horrid symbiosis as the two personalities merge into one.
As the mental states of the Eyeleech and its host coalesce, the resulting host-parasite gestalt will possess an intelligence greater than either of its component parts (although generally still inferior to that of the reasoning races, assuming the host is animal in nature). The Eyeleech will remain attached to the host's body with its mouth facing outward, to accept nourishment, and it will propel the host's body as its own, its eyes augmenting the host's eyesight just as the host's body augments the Eyeleech's physical capabilities.
Eyeleeches are so extraordinarily rare that little can be said of their habitat, except that it must be very secluded, and possibly have a very high level of background magic. No known recorded instance of an Eyeleech inhabiting a sentient host exists, but it is imagined that the results would range from "dangerous" to "catastrophically threatening" (depending on the host). Some very twisted individuals, driven to insanity by an extraordinary awe of Beholders and their kin and possessing the knowledge of the Eyeleech's existence, will seek out the creatures in order to offer themselves as hosts--however, no successful attempt at such has been recorded, indicating that either the would-be hosts died in the search, or did not possess a strong enough will to bend the Eyeleech's aims towards areas where history is known.