Shapechanger. The yuan-ti can use its action to polymorph into a Large snake, or back into its true form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It doesn't change form if it dies.

Innate Spellcasting (Abomination Form Only). The yuan-ti's innate spellcasting ability is Charisma (spell save DC 15). The yuan-ti can innately cast the following spells, requiring no material components:

Animal Friendship (Snakes Only). Casting Time (1 action), Range (30 feet), Components (V, S, M (a morsel of food)), Duration (24 hours): his spell lets you convince a beast that you mean it no harm. Choose a beast that you can see within range. It must see and hear you. If the beast’s Intelligence is 4 or higher, the spell fails. Otherwise, the beast must succeed on a Wisdom saving throw or be charmed by you for the spell’s duration. If you or one of your companions harms the target, the spell ends.

Fear. Casting Time (1 action), Range (Self (30-foot cone)), Components (V, S, M (a white feather or the heart of a hen)), Duration (Concentration, up to 1 minute): ou project a phantasmal image of a creature’s worst fears. Each creature in a 30-foot cone must succeed on a Wisdom saving throw or drop whatever it is holding and become frightened for the duration.
• While frightened by this spell, a creature must take the Dash action and move away from you by the safest available route on each of its turns, unless there is nowhere to move. If the creature ends its turn in a location where it doesn’t have line of sight to you, the creature can make a Wisdom saving throw. On a successful save, the spell ends for that creature.

Suggestion. Casting Time (1 action), Range (30 feet), Components (V, M (a snake’s tongue and either a bit of honeycomb or a drop of sweet oil)), Duration (Concentration, up to 8 hours): You suggest a course of activity (limited to a sentence or two) and magically influence a creature you can see within range that can hear and understand you. Creatures that can’t be charmed are immune to this effect. The suggestion must be worded in such a manner as to make the course of action sound reasonable. Asking the creature to stab itself, throw itself onto a spear, immolate itself, or do some other obviously harmful act ends the spell.
• The target must make a Wisdom saving throw. On a failed save, it purses the course of action you described to the best of its ability. The suggested course of action can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do.
• You can also specify conditions that will trigger a special activity during the duration. For example, you might suggest that a knight give her warhorse to the first beggar she meets. If the condition isn’t met before the spell expires, the activity isn’t preformed.
• If you or any of your companions damage the target, the spell ends.

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