Tiichi's Caustic Cataract

A stream of black sludge spouts from the ground in a 5-foot-radius, 20-foot high cylinder centered on a point within range. Each creature in the spell's area must make a Dexterity saving throw. On a failed save, a creature takes 4d8 acid damage, is knocked prone, and nonmagical armor worn by it is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10. On a successful save, the creature takes half as much damage and doesn't suffer from any other effects.

The black goo turns the spell's area on the ground into a slick surface until the end of your next turn. A creature that enters the area or ends its turn there must succeed on a Dexterity saving throw or fall prone.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each slot level above 3rd.