Corund's Caustic Coating

You coat a melee weapon with acid that lasts till the end of your next turn.  Until the end of your next turn, the first successful attack with that weapon does an extra (28) 6d8 acid damage. If the weapon is nonmagical or not made of adamantine, the weapon suffers a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed.

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.