According to its Monster Card, the ghost-like Mihstu, is an elemental from the para-elemental plane of Vapor. Which is a “shoulder plane” where the the planes of Earth and Water meet. They are described as solitary creatures, which is good, because their 8 hit dice combined with weapon resistance and constitution drain, makes just one of these monsters a formidable opponent.
The proper pronunciation for this creature, according to Frank Mentzer in Dragon Magazine #93, is MIS-too. So, the H is silent.
The art on this card is hard to pin down. I am not a good enough judge of style to say conclusively who worked on it. Which I guess is a testament to the talent of the various artists who worked on the Monster Cards. My only guess would be Jim Roslof. That would mostly be based on the way he tends to do backgrounds in other Monster Cards in this set. A two tone background with almost billowing gradient between the two shades.
This version of the Mihstu is otherworldly and exotic. A swirling green, white and blue alien presence that screams danger with its glowing starburst eyes and orange tipped tentacles. It is a magnificent work of fantasy art.
The Mihstu is one of the three new monsters in the first set of the Monster Cards. They would make their first book appearance in the Monster Manual II in 1983. The stats for the Mihstu card matche up nicely with its MM2 entry. So no surprises there. The art styles might differ, but this is one of those instances where the ink drawing in the Monster Manual II matches up nicely with Monster Card. Although in my opinion, the card is by far a superior representation.
The description on the card and the entry in MM2 have one striking difference. In the card they are from the para-elemental plane of Vapor, but in the book, which was published a year later, they are from the Elemental Plane of Air. So what happened during that year to cause such a big change?
In Dragon #73 (1983), Gary Gygax wrote a piece criticizing the way he had constructed the Elemental Planes in earlier editions. He would later go onto to simplify their geography. In doing so, the elemental planes of earth and water now meet at the Para-Elemental Plane of Ooze. This plane of slimes and ochre does not really seem a logical original point for the Mihstu, so they were moved to the Elemental Plane of Air.
That certainly simplifies things, but perhaps an equally good substitute would be the Quasi-Elemental Plane of Dust, which lies between the plane of Earth and the Negative Energy Plane.
The Mihstu is not a commonly used monster, but while rare, a pair of them make an appearance in Temple of Elemental Evil (1985) and I did manage to find a magic item that was created by Harold R. Powell in Dragon Magazine #117, that could be used to summon a Mihstu, the Ring of Vapor.
A Mihstu lair also makes an appearance in Dungeon #16 in an adventure by Martin and John Szinger called Palace in the Sky. While this adventure features wonderful art by Terry Dykstra, none of the illustrations are of a Mihstu.
The Mihstu is a tough underutilized monster, that in pairs, can take on a good-sized party of 7th level 1st edition characters. So DMs should use them sparingly, but they should use them. They are mysterious enough to keep your players guessing, while presenting a real challenge when it eventually comes down to combat.
Mihstu Random Encounter
Rumor has it that a local bachelor farmer was killed by a ghost. Locals are afraid to approach his place and say they hear and see weird things there at night. A cow that wandered into the field was reduced to bones overnight.
The problem is, this land is extremely fertile and valuable. So the town would like your group of adventurers to confront the ghost and banish it.
The ghost is of course a Mihstu. What the farmer didn’t know is that his farm lies on a portal to the elemental plane of wherever your Mihstus is from. The farmer did not realize this and tapped into it while digging a new hole for his outhouse. Now the party need to figure out how to defeat the Mihstu and anything else that crawled out of the outhouse and seal the hole.