Necrozma's body appears to be constructed out of black crystalline material; its disproportionately large arms have three claws jutting out of them, as well as a single large whitish crystal protruding backwards from each arm. Its small legs bend backwards like a bird's and have three toes each. A single small, hook-like wing grows out of each of its shoulders. The large protrusion attached to its head is its tail, which is white in the back with a dark gray symbol resembling a star in the center. This part of the tail glows while absorbing light that it converts to lasers. The sides of its head and the top of its tail are covered in spikes. Its face is actually a prism-like object known as the brain prism, which possesses a collection of multicolored shapes that resemble eyes. Despite being pitch-black, Necrozma's body constantly refracts light like a prism.
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 hosts' bodies are covered in parts of Necrozma's body, which are arranged like armor; in particular, Necrozma's large clawed arms are prominent in both forms, though they lack the large crystals jutting from them. Both forms have a triangular helmet with Necrozma's brain prism prominently visible, with bright glowing blue eyes for Dusk Mane Necrozma and pink eyes for Dawn Wings Necrozma. Using the split tail on Dusk Mane Necrozma's sides or Dawn Wings Necrozma's back, both of these forms can propel themselves forward using black light.
Once it absorbs enough light, Necrozma can undergo a process called Ultra Burst to reclaim its true form: Ultra Necrozma, a colossal, shining dragon armored by pieces of Necrozma's original form. Most of its body is made of pure light energy, as indicated by its non-solid shape; this light is produced from the brain prism, which is no longer visible. Ultra Necrozma has two pairs of clawed wings, a long, spiked tail, and a head resembling a shining star with several long horns on the sides and top. Its face is armored and has multicolored heterochromic eyes; the left eye is primarily red, while the right is mainly blue. In this form, Necrozma's large clawed arms and small legs have swapped places, respectively becoming Ultra Necrozma's feet and parts of its wings. As Ultra Necrozma, all of the pieces of Necrozma's base form have now become a shining golden color, though it will return to black if the light it has stolen escapes. Ultra Necrozma has a body temperature of 10,000 degrees Fahrenheit; 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.
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, including Solgaleo, Lunala, and Ultra Megalopolis; 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. While absorbing light, Necrozma's brain prism glows white.
Necrozma is the only known Pokémon capable of learning the moves Prismatic Laser and Photon Geyser, the latter of which can be upgraded into the exclusive Z-Move Light That Burns the Sky while in its Ultra Necrozma form. Dusk Mane Necrozma and Dawn Wings Necrozma can also learn Solgaleo and Lunala's signature moves Sunsteel Strike and Moongeist Beam, respectively, and upgrade them to their exclusive Z-Moves Searing Sunraze Smash and Menacing Moonraze Maelstrom.