Necrozma is a vaguely humanoid Pokémon whose body appears to be constructed out of a black crystalline material. The large protrusion attached to its head is its tail, which has a row of three large spikes along the top and is white in the back with a dark gray symbol resembling a star in the center. Its face is actually a prism-like object known as the brain prism, which possesses a collection of multicolored shapes that resemble eyes. While absorbing light, this brain prism glows white. Its thin arms have disproportionately large hands with three claws. Jutting out of the back of each hand is a whitish crystal that darkens toward the tip, and each elbow has a hook-like spike. Two large spikes jut from each shoulder and there is a row of smaller spikes along each side of its body. Its feet have three hook-like toes each.
Necrozma can take control of Solgaleo or Lunala by fusing with them, allowing it to assume its Dusk Mane Necrozma or Dawn Wings Necrozma forms, respectively. Because Necrozma is forcibly absorbing their light, Solgaleo and Lunala constantly glow in a manner similar to their Radiant Sun and Full Moon phases while fused. In both cases, the host's body is covered in parts of Necrozma's body, which is arranged like armor.
Dusk Mane Necrozma has armor around its paws, head, and sides. Necrozma's tail splits into two and attaches to the sides of Solgaleo's torso and its large claws extend from Solgaleo's back. The front feet have five black claws attached to it with two V-shaped crystals on top of it while the back foot has only three claws. It has a triangular helmet with a long, thin antenna on each side and Necrozma's brain prism prominently displayed. Solgaleo's own eyes and nose are visible as bright, blue lights. The host's tail gains a three black spikes attached to each other, as well as three spikes under the chin. A structure similar to one of Necrozma's legs extends from its back. Using the armor on its chest, it is able to propel itself by shooting black light.
Dawn Wings Necrozma has armor around its torso, head, and certain parts of its wings and tail. It also has a triangular helmet with the brain prism visible, as well as Lunala's own eyes visible as glowing pink lights. Necrozma's arms extend from the center of its chest. The armor surrounding its body has two large spikes jutting up from around its shoulders and four, glowing pink lights in the center. On its lower back are two thin, antennae-like growths extending downward. A structure similar to Necrozma's tail extends from the center of its back, and like its Dusk Mane form, can be used as a form of propulsion. There are eleven crystalline spikes attached around the body, four each on the wings, and three on Lunala's tail. These spines are capable of firing dark energy.
Once it absorbs enough light, Necrozma can undergo a process called Ultra Burst to reclaim its true form: Ultra Necrozma, a colossal dragon of pure light energy. Its size makes it the tallestPsychic-type. The various pieces of Necrozma's crystalline body are rearranged as a sort of "skeleton" for the rest of Ultra Necrozma's body. These pieces take on a golden glow from the light, but return to black without it. Its head is surrounded by several spikes and a pair of small wings, which causes it to resemble an eight-pointed star; there is also a small, lighter eight-pointed star on the back of its head. Its face is armored and has multicolored heterochromic eyes; the left eye is primarily red, while the right is mainly blue. It has two pairs of wings: one around its shoulders and one around its hips. While the wings themselves are made of light, the arm portions are made of its former legs and shoulders, respectively. Its former arms now cover its lower legs, and what was once its tail covers its chest. Finally, it has a long tail with a row of spikes along the top. Ultra Necrozma's body temperature exceeds 10,000 degrees Fahrenheit (5,540 degrees Celsius); anything that it touches will melt from the extreme heat. It can fire laser beams from every part of its body, and these lasers can reach over 18 miles away, burning through everything in their path. As seen in the anime, when Necrozma uses up all its light it enters into a deep slumber unless it absorbs more light.
Necrozma originates from Ultra Space and is capable of opening and traveling through Ultra Wormholes. In Ultra Megalopolis, it is known as the "Blinding One" (Japanese: かがやきさまRadiant One). It once had the ability to give light, but lost this ability after being injured under unknown circumstances; Professor Kukui and Lillie theorize that the Sparkling Stones used to perform Z-Moves are fragments of Necrozma's body that it lost upon this injury. In its incomplete state, Necrozma suffers constant pain and displays immense hostility. In an effort to restore itself to its original state, Necrozma steals light from other places and beings. This behavior caused it to be feared as the "pillager of light" in ancient Alola. When Necrozma absorbs light from a world, that world becomes shrouded in darkness until the stolen light is returned.
Ultra Necrozma debuted in Tough Guy Trials!, during a fantasy. Acerola showed Ash a book containing a legend detailing the origins of Alola, in which Ultra Necrozma, Solgaleo, and Lunala first brought light to the region. The ancient people dubbed Necrozma, who shined the brightest out of all three, the "Blinding One". Team Rocket also heard the story and contacted Giovanni about it, only to find out from Matori that he already knew of Necrozma and ended the transmission, much to their surprise.
Necrozma first appeared in PASM25. One appeared from an Ultra Wormhole and possessed Solgaleo to become its Dusk Mane form. It then grabbed Sun and dragged him into the wormhole with it, forcing Moon and Lunala to chase after them. It is worshiped by the people of the Ultra Megalopolis as the "Blinding One".
Ultra Necrozma is the only Pokémon with a base stat total of 754.
Necrozma has the most body styles of all Pokémon with a total of four between its different forms.
Necrozma's status as an Ultra Beast differs between media.
In the core series games, Necrozma is not an Ultra Beast. Beast Balls are ineffective against it and regular Poké Balls are normally effective against it. Additionally, Necrozma's Pokémon Sun Pokédex entry and Dulse clarify that it is similar to the Ultra Beasts, but not one of them.
In the anime, it, alongside the other members of the Light trio and their pre-evolved forms, are referred to as Ultra Beasts. Necrozma is given the codename of UB Black (Japanese: ＵＢ:ＢＬＡＣＫUB: Black).
In the TCG, Dusk Mane, Dawn Wings, and Ultra Necrozma are classified as Ultra Beasts, while its base form is not.
In Pokémon Duel, all forms of Necrozma are considered Ultra Beasts.
Necrozma is fought three times in the main story of a core series game, more than any other Legendary Pokémon.
Necrozma has access to the most exclusive Z-Crystals with a total of three.
Necrozma's Dusk Mane and Dawn Wings forms may be based on the concept of solar and lunar eclipses, respectively. The forms' alternate names in Japanese, 日食 nisshoku (solar eclipse) and 月食 gesshoku (lunar eclipse) can also be literally translated as "eating the sun/moon", reflecting how Necrozma absorbs Solgaleo or Lunala to assume those forms. Its Ultra Necrozma form may be based on giant stars or supernovae. Design-wise, Dusk Mane Necrozma and Dawn Wings Necrozma retain Solgaleo's lion and Lunala's bat bases, respectively, while Ultra Necrozma resembles a four-winged wyvern. The fact that Necrozma transforms into a Dragon-type could be based on legends about Kihawahine, a Hawaiian dragon goddess.
Necrozma may be a combination of νεκρός nekrós (Ancient Greek for dead), 黒 kuro (black), and πρῖσμα prîsma (Ancient Greek for prism). It may also involve Project Ozma, an experiment made to search for extraterrestrial life.