The Tavern

Level 6 Hexes/Conjurations/Air or level 7 Hexes/Conjurations, makes short blades viable in end game.
The cloud is somewhat similar to Poisonous Cloud except it "poisons" AC instead of HP: every turn the monster is in cloud, it loses 1 AC temporarily. AC is slowly restored back to normal level like is done with player's AC after corrosion attack.

Rod of Clouds already has this effect, it's very very strong.

It is not reliable and requires high investment in Evocations, also I wanted to add yet another reason to train hexes higher.

I think the point of allowing this to be a spell instead of only randomly from a rod is that then the player could build around it, and it would work well with things like short blade melee. And short blade users tend to have hexes to some degree anyways, so this would be a good fit for stabbers dealing with things they can't stab. I'd say let the cloud do some damage as well but significantly lower than poison/freezing cloud, although this is debatable. Acid resistance is very rare and so this would be a lot less resistible than poison/freezing cloud, so it should be numerically weaker.

I suggest the cloud to deal no damage, otherwise it would be too similar to Cloud Cone or Poisonous Cloud. It's a pure debuff spell.