Mihstu - CR 9 (Medium Elemental)

Another horror I too regularly unleashed on my players back in the days of playing AD&D 1st Ed and 2nd Ed, the Mihstu here is converted from the one found in the AD&D 1E Monster Manual II. 

Mihstu's are vaporous elementals of air, that are generally quite eager to avoid trouble. However, when trapped, or magically compelled, they are surprisingly vicious, being both difficult to hurt, and packing quite a punch.

In the version I used as my base, the monster could envelop and drain the life from a victim. I thought this was a bit clunky, especially given its vaporous form, and went for the drain happening with each hit with its tendrils. I also wanted to keep the terrifying feel of it draining constitution, without making the maths too heavy (although I do still use a lot of ability score draining etc in my games).

Wooo! I am 9/10th soap suds, 1/10th pure eeeeeevil!