Corrosive Blade

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails.  This attack is a critical strike on a roll of 19 or 20 and deals an additional 1d6 acid damage.  If a creature suffers a critical hit from this attack, their Armor Class is reduced by 1 until the end of your next turn.

At Higher Levels.  At 5 level, the melee attack deals an extra 1d6 acid damage and the Armor Class reduction on a critical hit increases by 1.  The damage increases again by 1d6 and Armor Class reduction on a critical hit by 1 at 11level and 17 level.