Ondine's curse. If the undine is decived, the deciver is cursed to have to manually do every action such as breathing, not allowing the usage of bonus actions. Remove Curse or other magic breaks the curse.

Spellcasting. The Undine is a 3rd-level spellcaster. Its spellcasting ability is [ability score] (spell save DC 13, +5 to hit with spell attacks). The Undine has following Sorcerer spells prepared:
Cantrips (at will): Acid Splash, Frostbite, Shape Water
1st level (4 slots): Absorb Elements, Ice Knife
2nd level (2 slots): Invisibility, See Invisibility

Rapier. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 5 (1d8 + 1) piercing damage. 

Dart. RangedWeapon Attack: +3 to hit, range 20/60 ft., 1 target. Hit: 3 (1d4 + 1) piercing damage.