It is not known to evolve into or from any other Pokémon.
Marshadow was officially revealed on April 7, 2017.
Artwork of Zenith Marshadow
Marshadow is a humanoid Pokémon that appears to be made of smoky gray shadows. The area around its face resembles headgear, such as a helmet. On top of the helmet are two curled horns or ears with a wispy horn between them. Its eyes are orange-red with yellow pupils, and above each eye is a yellow, oval marking. Each arm has a small bump on the wrist. Around its neck is a smoky collar and there is a trail of shadows on each foot.
When angered or attacking, such as using its exclusive Z-Move, Marshadow's fighting spirit will begin to burn, causing it to become Zenith Marshadow. In this state, its headgear, wrist bumps, and collar flare up and resemble green and yellow flames. Marshadow's eyes also flare up when in this state, but do not change in color.
Very few people have seen it, so its existence was the stuff of legends and myths. A cautious Pokémon, Marshadow sinks into the shadows to ensure it is not noticed while it observes its surroundings. While in hiding in the shadows of others, it can understand their feelings and copies their abilities. Through mimicry, it will eventually become more powerful than those it imitates. As seen in the anime, Marshadow is also capable of causing nightmares. Marshadow is the only known Pokémon capable of learning the move Spectral Thief and exclusive Z-MoveSoul-Stealing 7-Star Strike.
Marshadow debuted in a major role in I Choose You!, where it was revealed to be a guide from the shadows who leads those chosen by Ho-Oh to be the Rainbow Hero.
Marshadow made its main series debut in The Dex Can't Help It!, where Rotom wanted to record its data, but couldn't due to it not being inside its Pokédex. Another Rotom inhabiting the Pokédex managed to take a blurry picture of the Marshadow, but deleted it afterwards.