Acid Rain

Until the spell ends,  harsh acidic rain falls in a 20-foot-tall cylinder with a 40-foot radius centered on a point you choose within range. The area is lightly obscured, and exposed non-magical flames in the area are doused. Creatures within the area that are exposed to the rain must make a Constitution saving throw or take 2d8 acid damage and become afflicted by corrosion. Creatures that begin their turn in the area must make the saving throw at the beginning of each of their turns. Creatures that are already affected by corrosion do not take additional damage after the first failed save, but instead take additional stacks of corrosion if possible on additional failed saves.

Corrosion- Target takes (1d8) acid damage at the beginning of each of its turns equal to one hit die of the damage that triggered it and a -1 penalty to its AC. This effect stacks each time it triggers without being removed, up to three times (3d8 acid damage and a -3 penalty to AC). Upon being removed, the AC penalty is immediately negated. The AC penalty has no effect against AC granted by a spell or other nonphysical force (shield, shield of faith, mage armor, etc)