Necrozma did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Necrozma is a pitch black Pokémon with a crystalline body. One of its most defining features is its large hands and claws, with a white triangle on its palms and a white-to-black crystal pillar protruding from each back. The thin arms that have these large paws attached also have angular hooks on each elbow. Its avian-like legs also feature three hooked claws that point upwards. Its body appears as if it's wearing a spiked coat. Necrozma also has a large triangular protrusion on its head, which is topped with three spikes and has a white, rhombus-shaped backside with a gray eight-pointed star on it. Its face consists of many triangular shapes in various colors, patterned in a way it resembles eyes and a mouth.
Dusk Mane Necrozma takes the form of a yellow-glowing Solgaleo with the black prisms shaped like Necrozma's body parts. Three black spikes appear on the chin and three more claws appear visible on the paws, with the front paws also being decorated with more black prisms. Prisms that resemble Necrozma's head protrusion appear on its back, as well as a pair of claws resembling its hands on top of it. Finally, on its face is a visor different from Solgaleo's, being a black, translucent triangle with two antenna-like protrusions from each side. In the center of the visor lies a colorful triangle.
Dawn Wings Necrozma takes the form of a blue-glowing Lunala, again with Necrozma's black prisms. Four pairs of prisms can be seen on the wings, and three more on the tail. The chest is entirely enveloped by a black prism shaped like a V. Two large claws are attached to this prism, as well as two thin legs akin to Dusk Mane Necrozma's antennae. The head protrusion of Necrozma can be seen on Dawn Wings Necrozma's back. Finally, a black prism replaces Lunala's regular visor, and has a colorful triangle inside, just like its Dusk Mane counterpart.
Necrozma is much alike an Ultra Beast, but has not been confirmed to be one at this point. It has been found asleep deep underground, and is thought to have come from another world in ancient times. Despite being black, it can refract light like an actual prism. Light has been confirmed as its source of energy, which it uses to fire off laser beams in any direction.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Necrozma is possibly based on the concept of prisms and light refraction.
Due to the fact that its head protrusion's back features a star, its name is a pun on νεκρός nekrós (death), and that it is black, it could also be based on a black dwarf, a theoretical dead star that no longer emits light or heat. Given the fact that it also absorbs light, it could also be based on a black hole
Necrozma's name may be a combination of νεκρός nekrós (Ancient Greek for death), 黒 kuro (black), and πρῖσμα prîsma (Ancient Greek for prism). It may have also taken inspiration from Project Ozma.