Scale Rot

You summon death and decay to plague your enemies. For dragons, this act often takes the form of attacking a foe’s armor and scales, as a way of weakening an enemy dragon and leaving it plagued by self-doubt and fear. (This enchantment is useful against any armored creature, not just dragons.)

One creature of your choice within range that has natural armor must make a Constitution saving throw. If it fails, attacks against that creature’s Armor Class are made with advantage, and the creature can’t regain hit points through any means while the spell remains in effect. An affected creature can end the spell by making a successful Constitution saving throw, which also makes the creature immune to further castings of scale rot for 24 hours.