Shapechanger. The yuan-ti can use its action to polymorph into a Medium 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. If it dies, it stays in its current form.

Innate Spellcasting (Yuan-ti Form Only). The yuan-ti’s innate spellcasting ability is Charisma (spell save DC 13). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)

3/day: suggestion

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

Spellcasting (Yuan-ti Form Only). The yuan-ti is a 10th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following paladin spells:

1st (4 slots): detect evil and good, divine favor, searing smite, thunderous smite

2nd (3 slots): branding smite, magic weapon, lesser restoration

3rd (2 slots): blinding smite, dispel magic

Sseth’s Blessing. When the yuan-ti reduces an enemy to 0 hit points, the yuan-ti gains 9 temporary hit points.

Corrupting Smite: When The Leech hits a target with a melee attack he can expend one spell slot to deal acid damage to the target, in addition to the weapon's damage. The extra damage is 2d8 for a 1st-level spell slot, plus 1d8 for each spell slot higher than 1st, to a maximum of 3d8.

Aura of Corruption: Any non-friendly creature within 10 feet of The Leech must make a Charisma saving throw (DC 16) or suffer Disadvantage on all saving throws while within the Aura of Corruption.  The saving throw can be attempted at the end of the non-friendly creature's turn. Once a creature is successful on the saving throw this ability will no longer affect them.

Corrosive Gift: When The Leech hits a creature with a melee attack, the creature takes an extra 1d8 acid damage.

Multiattack (Yuan-ti Form Only). The yuan-ti makes one bite attack and one scimitar attack or two scimitar attacks.

Corrosive Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 8 (1d4 + 6) piercing damage plus 50 (10d10) acid damage.

Scimitar (Yuan-ti Form Only). Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 11 (1d6 + 8) slashing damage plus 4 (1d8) acid damage.