Mismagius is a purple, ghost-like Pokémon. It has a round head with elaborate tufts resembling a witch's hat atop it. The tips of the tufts are paler than most of the body. It has a thin, red mouth that resembles the letter "W." Below its head, Mismagius has a long neck with a round lump midway down. Three, elliptical red spheres surround the upper portion of Mismagius's body. There are extensions on the sides of its lower body with the foremost ones tattered compared to rear ones. This gives Mismagius the appearance of wearing some kind of cloak.
If it is powerful, Mismagius can use advanced incantations for various purposes, such as causing misery or happiness. People that hear its cries will usually receive headaches and hallucinations. As shown in the anime, Mismagius can put its victims to sleep, and cause them to enter a strange dream world without their noticing. If Mismagius falls asleep or is knocked out, the victim will wake up.
A Mismagius appeared in Zoroark: Master of Illusions, under the ownership of Grings Kodai, who has owned it since it was a Misdreavus. Years ago, when Kodai first discovered the Time Ripple, Misdreavus was present at the scene. After evolving, Mismagius still served Kodai faithfully and helped him by using its Psychic to prevent Ash from stopping Kodai from taking the energy from the Time Ripple and renewing his ability to see the future. Mismagius was later defeated by Pikachu's Volt Tackle.
Mismagius was one of the Pokémon Lily used while with Team Galactic. This Pokémon was used to keep Lily hypnotized and under Team Galactic's control.
In the Pokémon Adventures manga
Fantina owns a Mismagius that first appeared in Perturbed by Pachirisu. Platinum later battled Mismagius in her Gym battle against Fantina. She was very powerful and shrewdly manipulative using her illusion attacks, and using Trick Room to use Empoleon's speed against her, defeated both Empoleon and her Ponyta before Diamond's Lax finally took her down at the last minute.
Mismagius's base HP, Attack, and Defense are all the same as Misdreavus's, but its base Special Attack, Special Defense and Speed are higher.
Mismagius is based on the concept of mages and witches. It also shares similarities with banshees. Mismagius's coloration might be a reference to the Nasu-babā, a witch-like yokai with a distinct purple skin.
Mismagius may be a combination of mis- (bad, wrong) and magus or magic.
Mumargi may be combination of 夢魔 muma (nightmare) and mage or magic.