Devil's Sight. Magical darkness does not impede the yuan-ti's darkvision.

Magic Resistance. The Yuan-Ti Bladesinger has advantage on saving throws against spells and other magical effects.

Multiattack. The Yuan-Ti Bladesinger makes five attacks: four with it's weapons, and one with it's tail.

Scimitar. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) slashing damage.

Whip. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 6 (1d4 + 4) slashing damage. On a successful hit, the target must make a DC 13 Dexterity save or be pulled 5 ft. closer to the Yuan-Ti Bladesinger.

Tail. Melee Attack: +7 to hit, reach 10 ft., one creature. Hit: 9 (1d10 + 4) bludgeoning damage. If the target is Medium or smaller, it is grappled (escape DC 13). Until this grapple ends, the target is restrained, the Yuan-Ti Bladesinger can automatically hit the target with its tail, and the creature can't make tail attacks against other targets.

Spellcasting (Only while Bladesong is active). The Yuan-Ti Bladesinger is a 3rd-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +5 to hit with spell attacks). The Yuan-Ti Bladesinger has following spells prepared:
Cantrips (at will): Firebolt, Mold Earth, Poison Spray
1st level (4 slots): Bane, Catapult, Searing Smite, Jump
2nd level (3 slots): Misty Step, Dust Devil
3rd level (2 slots): Fireball, Erupting Earth

Bladesong (2/Day). The Yuan-Ti Bladesinger can use a bonus action to start the Bladesong, which lasts for 1 minute. It ends early if the Yuan-Ti Bladesinger is incapacitated, if the creature is disarmed of their weapons, or if the creature uses two hands to make an attack with a weapon. The Yuan-Ti Bladesinger can also dismiss the Bladesong at any time (no action required).

While Bladesong is active, the creature gain the following benefits:

The creature gains a bonus to your AC equal to its Intelligence modifier (+2)
The creature's walking speed increases by 10 feet.
The creature has advantage on Dexterity (Acrobatics) checks.
When using multiattack, the creature can replace one of its weapon attacks with a cantrip.

Cunning Action. The Yuan-Ti Bladesinger can use a bonus action to dash, disengage, or hide.

Parry (Only while Bladesong is active). The Yuan-Ti Bladesinger adds +5 to it's AC against one melee attack that would hit it.