Nihilego are mind-bogglingly abundant in the Deep Sea. Much of the dimension is overgrown with glassy reefs built up over years of Nihilego reproduction and activity. Nihilego's body is composed of an amorphous vitreous substance of a translucent blue-white hue. Though it can change shape at will, it always possesses twelve tentacles, three oral palps, a transparent bell, a blue loop of sensory tissue, and two globular organs that sense acceleration in order to maintain balance. The frosted star-shaped patterns on its bell have been determined to be the equivalent of neurons.
The shape most commonly seen resembles a jellyfish mimicking the shape of a humanoid girl in formal wear. The bell remains at the top of the creature as usual, ringed by the sensory loop in a way that resembles a frill of lace. Together they resemble a sun hat. Four tentacles shift forward to serve as manipulators, two on each side of the oral palps. The rest of the tentacles shift to the back of the creature, where they fuse for most of their length to give the impression of a dress. The outermost tentacles of the "dress" contain the balance organs, and the innermost tentacles stretch downward in an imitation of legs. While in this form, Nihilego's mannerisms resemble that of a quiet human child, believed to be its default behavior.
Nihilego's mind is extremely difficult to study. All standard psi detection devices have come up blank, and observations in the wild have suggested the presence of basal instincts in addition to very faint awareness of its surroundings. It is believed that Nihilego's strange psychology is an adaptation to protect itself from the psychic powers of its main predator, Yolawis. When left alone in safe conditions, Nihilego filter-feed from their surroundings and drift aimlessly. When presented with others of their kind, Nihilego revert to their childlike shape and interact like human children, interacting with their manipulator tentacles, hugging, "dancing", and passing rocks and crystals between themselves. The purpose of these behaviors is unknown and is the topic of heated debate amongst laboratory scientists assigned to analyze the Divers' data.
Nihilego appear to reproduce asexually. Individuals will occasionally spawn when completely alone, then immediately vacate the area. The cloud of eggs is carried to new areas by currents, giving enough space for each egg to develop without much competition from its siblings. Nihilego eggs are fragile, tiny, and highly sensitive to their environments, making them extremely difficult if not impossible to raise in captivity. The eggs require the spacetime structure of their homeworld. Attempts to recreate this spatial structure in a laboratory have failed.
Nihilego's feeding habits appear to be parasitic in nature. Though it can consume particulate substances from the environment through its skin, it obtains most of its nourishment by latching onto other organisms, typically by wrapping its bell around them. Nihilego will change shape to accommodate the physiology of its target. The oral palps will mold onto the surface of the host's body in order to draw nutrients out of it. Nihilego is able to derive nutrients from its host with simple skin contact.
It then pumps a powerful neurotoxin into its host. Nihilego delivers its venom through nematocyst-like structures in its tentacles and palps. Despite being composed of the same nonatomic substances as the rest of Ultra Deep Sea, its effects are the same regardless of the victim's chemical and physical makeup. Instead of damaging, Nihilego venom is psychoactive. Initial envenomation lowers the victim's inhibitions, intensifying reactions and reducing capacity for metacognition. The venom amplifies the host's motivation to fulfill its own needs and wants. In the wild, this takes the form of ravenous feeding patterns in the host to provide enough sustenance for both individuals. When Nihilego has eaten its fill, it will release its host and continue on its way as the venom slowly works its way out of the former host's system. Multiple Nihilego will parasitize the same host in succession in order to take advantage of the venom the host has yet to expel.
The end stage of envenomation involves the complete suppression of independent thought and a monomaniacal focus on providing for oneself and Nihilego. It is entirely capable of infesting sapient hosts, but the envenomation process takes much longer, and targets are able to pull away before they lose awareness of themselves. Only through prolonged exposure to Nihilego's neurotoxin can a highly intelligent organism come under its control. As such, laboratory study of captured specimens must be done with utmost caution. Rotating staff and providing the specimen with ample food are highly effective in preventing the envenomation of researchers.
When controlling a sapient host, a Nihilego's appearance can change drastically. The internal structure of its jelly crystallizes and darkens, refracting light in a way that produces prismatic flecks of color. The bell grows to envelop more of the host. Hard, opaque structures sprout from the bell to surround the tentacles, and the tentacles themselves can grow fingers, spikes, additional accelerometer globules, and even complex eyes. This phase is referred to as the possessive phase, as the desires, instincts, and personality of Nihilego and its host merge. No longer does Nihilego control its host. It is its host, and it bends to the host's desires as much as the host bends to the influence of the poison. These fused creatures are highly dangerous and very powerful. Only psionic disruption can split the two, and as such the Divers must train their assigned Solgaleo or Lunala to separate merged hosts in the event of an emergency.