Innate Spellcasting. The yuan-ti storm cultist innate spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: animal friendship, thunderwave, eldritch blast

3/day each: fear, lightning bolt

1/day each: greater invisibility

Magic Resistance. The yuan-ti has advantage on saving throws against spells and other magical effects.

Thunder Serpent's Wings (1/day). As a action the Yuan-ti Storm Cultist can burst a set of wings made of blue flames from its back. For 1 hour the Yuan-ti Storm Cultist has a flying speed equal to its walking speed. Al creatures within 5ft of the Yuan-ti Storm Cultist must make a DC 16 strength save, on a failed save the creature takes 7 (2d6) thunder damage and is knocked 10ft back. Nothing happens on a successful save. 

Multiattack. The yuan-ti storm cultist makes three melee attacks, but can use its bite and constrict attacks only once each. 

Storm Lance. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 8 (1d8 + 4) piercing damage plus 2 (1d4) lightning damage.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage plus 5 (1d10) lightning damage.

Constrict. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage, and the target is grappled (escape DC 14). Until this grapple ends, the target restrained, and the yuan-ti storm cultist can't constrict another target.