Ninja (5e Creature)
From D&D Wiki
Medium humanoid (any race), any alignment
Weakening Strike (1/Turn). When the ninja hits a target with a dagger attack on its turn, the target must make a DC 12 Constitution saving throw. On a failed save, starting at the beginning of the next turn, attack rolls against the target have advantage and the target has disadvantage on saving throws against damaging spells until the end of the ninja's next turn.
Dagger. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.