Nihilid are the polyp stage of Nihilego, mirroring the life cycles of Earthlike jellyfish Pokémon. When a Nihilego spawns, the eggs drift to the seafloor, take root, and eventually hatch into a bundle of short tentacles. Over time, it builds a vitreous carapace of upward-pointing crystal scales. Each scale protects a single tentacle, which draws small particles of food into the central body. It is not uncommon for a Nihilid to accidentally cannibalize its neighbors' ephyrae while it filter feeds. When threatened, Nihilid will retract all its tentacles into its scales. As the polyp reaches its final stages of growth, the scales fuse into a single conical stone, protecting the vulnerable body of a newborn Nihilego.
Over its lifetime, a single Nihilid produces several Nihil Ephyrae, which mature into adult Nihilego over the course of six months. Under safe conditions, an ephyra may grow instantaneously into a Nihilego in a process comparable to standard evolution. After several ephyrae are released from Nihilid's primary body, its living tissue will rise into a transparent bubble and exit its glassy shell, evolving into Nihilego via the standard method. The conical stone shell is left behind. Many generations have produced towering shelves of old Nihilid skeletons, forming crystal reefs where a number of Ultra Beasts hide and live.
While Nihilego's sentience is in question, Nihilid has been confirmed to possess no mental processes more advanced than reflexes. Trainers struggle to give it orders in battle as it often doesn't respond at all, though it will raise its defenses, cast Reflect and Light Screen, and Protect on its own. Because of Nihilid's sessile nature, it learns no attacking moves whatsoever, being limited to Harden, Protect, Safeguard, Mirror Coat, and barrier moves. According to studies, its "mind" is so weak and vestigial that it is susceptible to psychic suggestion even from non-psionic humans. This is the only way to issue it commands in a Pokémon battle, making it difficult to train a captive Nihilid for evolution.