Amphibious. The yuan-ti captain can breathe air and water.

Spellcasting. The yuan-ti captain is a 4-level spellcaster. Its spellcasting ability is Wisdom (spell save DC16, +8 to hit with spell attacks). The yuan-ti captain has following druid spells prepared:

Cantrips (at will): frostbite, poison spray, shape water

1st level (4 slots): create or destroy water, entangle, thunder wave

2nd level (3 slots): flame blade, warding wind

Multiattack. The yuan-ti captain makes three attacks; one with it's bit and two with it's longsword.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 14 (2d8+5) piercing damage, plus the target must succeed on a DC19 Constitution save or take 13 (3d8) poison damage.

Longsword. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 16 (2d10+5) slashing damage, plus the target must succeed on a DC17 Strength save or be knocked prone.