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

Improved Initiative. Zanth gains a +5 bonus to initiative checks.

Sneak Attack. Once per turn, Zanth can deal an extra 7d6 damage to one creature he hits with an attack with a finesse or ranged weapon if he has advantage on the attack roll.  He doesn't need advantage on the roll if another enemy of the target is within 5 ft. of it, that enemy isn't incapacitated, and he doesn't have disadvantage on the attack roll.

Innate Spellcasting. Zanth is a 13th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). Zanth has following spells prepared:
3/day each: darkness, poison spray, unholy aura
1/day each: contagion, unhallow, unholy blight, web (Cloak of Arachnida, DC 13)

Multiattack. Zanth makes two attacks: either one each with bite and claw, two with short sword, or two with shortbow.

Bite. Melee Weapon Attack: +7, to: hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage. 

Claw. Melee Weapon Attack: +10, to: hit, reach 5 ft., one target. Hit: 7 (1d4 + 5) slashing damage.

Masterwork Short Sword. Melee Weapon Attack: +11, to: hit, reach 5 ft., one target. Hit: 9 (1d6 + 6) slashing damage.

Shock Shortbow. Ranged Weapon Attack: +11 to hit, range 80/320 ft., one target. Hit: 9 (1d6 + 6) piercing damage plus 4 (1d6) lightning damage plus 7 (2d6) necrotic damage.  On a hit, the creature must succeed on a DC 15 Wisdom saving throw on have disadvantage on all attacks until the end of its next turn.

Cunning Action. Zanth can take a bonus action on each of his turns to take the Dash, Disengage, or Hide action.

Uncanny Dodge. When an attacker that Zanth can see hits him with an attack, he can use his reaction to halve the attack's damage.