You cause acidic rain to fall heavily in a region by conjuring a storm cloud in the shape of a Cylinder that is 4m tall with a radius of 12m. The cloud is centred on a point you can see 30m directly above you. The spell fails if you can’t see a point of air where the cloud could appear (for example in a room that can’t accommodate the cloud).

The area becomes Lightly Obscured and flames in the area are doused.

When a creature enters the spell's area for the first time on a turn or starts its turn there, it must make a Constitution saving throw, taking 2d10 Acid damage and potentially receiving stacks of

Sap

Critical Success (+10). The target is unaffected.
Success. The target takes half damage.
Failure. The target takes full damage and receives 2 stacks of Sap.
Critical Failure (–10). The target takes 1.5× damage, and receives 5 stacks of Sap.

Either way, any nonmagickal metal armour or metal shields that they are wearing takes a permanent and cumulative –1 penalty to the AC it offers. Armour reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed.

Objects that are vulnerable to corrosion that aren't being worn or carried automatically corrode if they are in the area.