Magic Resistance. The Yuan-ti drops the DC of a spell when it or another Yuan-ti within five feet of it by four.

Spellcasting. The assassin is a 4-level spellcaster. Its spellcasting ability is charisma (spell save DC 17, +4 to hit with spell attacks). The assassin has following  spells prepared requiring no material components :
Cantrips (at will): eldritch blast , firebolt, ray of frost
1st level (4 slots): animal friendship (reptiles only), disguise self sleep

Multiattack. The Yuan-ti makes three attacks.

Serpent's fang longsword. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 10 (2d6 + 2) [slashing] damage plus 8 (1d6 + 3) poison damage. 

Shortbow. RangedWeapon Attack: +3 to hit, range 50/180 ft., 1 target. Hit: 6 (1d6 + 2) [piercing] damage plus 9 (1d10 +4) [poison] damage. 

Jump. If the Yuan-ti is hit with a melee attack from a non magical weapon the yuan-ti only takes half damage.