Skeletons who are disintegrated by magical force like fire, lightning, and especially skeletons reduced to crusty frozen microparticles, can be raised in noncorporeal fashion.
Skeleton Clouds are clouds made out of one or more skeletons who were animated magically and later de-substantiated. Their health, attack values, and other particulars are exactly as a Skeleton would be in your game with four exceptions:
- They are not just resistant to piercing and slashing damage they in fact take no damage from physical attacks except from magic items, when they take normal damage. This cuts both ways and they cannot wield weapons or affect you physically without being fortunate enough to have a special power (below).
- They are immune to being turned by Clerics and holy types under Name Level.
- They can do anything that fog can do including get through doors and cracks and keyholes and stuff.
- 10% have the ability to inflict a static charge on anybody passing through them, doing 1d4 damage, half on a Petrify save. 10% can inflict 1d4 cold damage with their spectral touch, half on a Petrify save. 5% can do both. All Skeleton Clouds leave you covered in dusty, sooty moisture. Scrolls and spellbooks have a 5% chance of running (cumulative for every Skeleton Cloud affecting you this round) and all torches/lanterns are doused and all firestarting equipment is too wet to work.
The best way to fight them is to scatter them with some elemental breeze magic, dispel them over time (like Insect Swarms), or throw some sawdust or some flammable aerosol in them and watch their natural static charge blow them up. That is incredibly fun.
Finally, why not just air elementals? Because air elementals are always terrible and never shaped like skeletons and not made of thousands of dead people they're just from a universe where breezes act like pissy teenagers.
Skeleton Clouds can also rain blood but this has no effect other than giving me A HUGE ERECTION.