Assassinate. During its first turn, Zeerith has advantage on attack rolls against any creature that hasn't taken a turn. Any hit Zeerith scores against a surprised creature is a critical hit.

Fey Ancestry. Zeerith has advantage on saving throws against being charmed, and magic can't put him to sleep

Innate Spellcasting. Zeerith's Innate spellcasting ability is Charisma (spell save DC 12 +4 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

At will: dancing lights 1/day: darkness faerie fire levitate (self only)

Evasion. If Zeerith is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, he instead takes no damage if it succeeds on the saving throw, and only half damage if he fails.

Sneak Attack. Once per turn, Zeerith deals an extra 14 (5d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the assassin that isn't incapacitated and he doesn't have disadvantage on the attack roll.

Sunlight Sensitivity. While in sunlight, the drow has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Multiattack. Zeerith makes up to two ranged attacks or three melee attacks, or one of each

Trick. Melee or Ranged Weapon Attack: +10 to hit, reach 5 ft. or range 30/90 ft. one target. Hit: 8 (1d4 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (4d6) poison damage on a failed save, or half as much damage on a successful one.

Treat. Melee or Ranged Weapon Attack: +10 to hit, reach 5 ft. or range 30/90 ft. one target. Hit: 8 (1d4 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 12 (4d6) poison damage on a failed save, or half as much damage on a successful one. Whenever you make a critical hit with this dagger, you regain an amount of hit points equal to half the damage you deal with that attack.

Enchanted Repeating Hand Crossbow. Ranged Weapon Attack: +10 to hit, range 30/120 ft., one target. Hit: 9 (1d6 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 15 (5d6) poison damage on a failed save, or half as much damage on a successful one.