Anyone have any idea what's next for these?
Their attach reflection ability?
Misi do not exist in three of the four dimensions that humans and humanoids are capable of sensing. They have no physical manifestations in terms of length, width, or height. Instead, they exist solely in the fourth through sixth dimensions. Consequently, they cannot be injured by any form of physical attack, although they can be contacted and affected by magic.
Occasionally, misi can be glimpsed in the spacial dimensions as rainbow-colored scintillations of indistinct shape, but for no more than a few moments at a time. Such apparences can occur anyplace – on the surface of a planet, in the depths of wildspace, or even out on the phlogiston ocean. However, mist are almost always seen near spelljammer ships, for they are attracted to the magical emanations associated with the spacefaring craft.
Here it is...
So essentially they're ethereal, but those who can reach the Ethereal Plane have no special bonus against them? Only those who can somehow reach the "4th-6th dimesions"?
Or should we just simplify things and make them from the Ethereal Plane?
Isn't it closest to being on the material plane but incorporeal? I'd just do that. Since the original doesn't specify how a magical weapon (partly a physical attack, partly a magical attack) acts on them, I think we're free to specify. We can just state in flavor that they are on a different plane, but it acts like being normally incorporeal.
The misi cannot be injured by any sort of physical attack, including those made with magical weapons. They are vulnerable only to damage from spells, and then only if an individual misi can be located to have the spell cast at it.
Characters attempting to locate a misi have a percentage chance equal to three times their Intelligence score of spotting a misi’s scintillating body for one round. For example, a character with an Intelligence score of 10 has a 30% chance of spying a misi, and a character with a Intelligence of 18 has a 54% chance. Misi cannot be detected by means of a detect invisible spell, but they can be located by use of a locate animal, locate object, or ESP spell.
Definitely immunity to physical attacks. The miss chance almost sounds like a blink spell.