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 short, thorax-like lump from which its body connects. 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.
Mismagius usually shows up without warning to cause havoc on unsuspecting humans or Pokémon. If it is powerful, Mismagius can use advanced incantations for various purposes, such as causing misery or happiness. Due to this, people often seek to capture Mismagius as if their life depends on it, solely to make someone fall in love with them. There is a custom on inviting Mismagius to one's home, usually to ward off misfortune with its spells. However, this can also lead to disaster should Mismagius ever be provoked. 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. It can sometimes be found roaming around lakes and mountainsides at night.
In Zoroark: Master of Illusions, Grings Kodai has owned a Mismagius since it was a Misdreavus. It 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.
In The Semi-Final Frontier!, a Trainer's Mismagius went up against Tobias during the Lily of the Valley Conference finals. It battled his Darkrai off-screen, but as with the other five Pokémon owned by its Trainer, it was easily defeated by the Pitch-Black Pokémon.
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 seems to be based on the concept of mages and witches. Mismagius's coloration might be a reference to the Nasu-babā, a witch-like yōkai with a distinct purple skin. Its shape also resembles the Teru teru bōzu. Its Pokédex descriptions of its cries sounding like incantations and tormenting people are reminiscent of curses. Another possible source of origin is the banshee, a female spirit from Irish mythology, known in modern media for its unearthly wailing and screaming.
Its names in different languages referencing dreams also points to Mares as a possible inspiration.
Mismagius may be a combination of mischievous or Misdreavus and magus or magic.
Mumargi may be a combination of 夢魔 muma (nightmare; demon in a dream) and 蠱 maji (to bewitch), mage, or magic.