Ambush Viper

Camouflage. The Ambush Viper has advantage on Dexterity (Stealth) checks and on attack rolls against creatures that fail a DC 17 Wisdom (Perception) check with disadvantage.

Built in Spells. The Ambush Viper automatically knows Bestow Curse and Blindness/Deafness and can cast both spells 4/day.

Surprise Attack. If the Ambush Viper surprises a creature and hits it with an attack, the target takes an extra 18 (6d6) poison damage from the attack.

Spellcasting. The Ambush Viper is a 4th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 16, +8 to hit with spell attacks). The Ambush Viper has the following Wizard spells prepared:
Cantrips (at will): Acid Splash, Minor Illusion, Poison Spray, True Strike
1st level (4 slots): False Life, Magic Missile, Thunderwave

2nd level (3 slots): Hold Person, Melf's Acid Arrow

Multiattack. The Ambush Viper makes three attacks and casts one spell, one attack with it's Bite, one attack with it's Tail Spike, and one attack with it's Metal Pin.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (3d6 + 7) piercing damage and the target must make a DC 16 Constitution saving throw (the target makes it with disadvantage if they're surprised) taking 15 (3d10) poison damage and are poisoned. On a success the damage is halved and the target is not poisoned.

Tail Spike. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 9 (1d4 + 7) piercing damage and the target must make a DC 16 Constitution saving throw (the target makes it with disadvantage if they're surprised) taking 24 (4d12) poison damage are poisoned and suffer one level of exhaustion. On a success the damage is halved and the target does not suffer a level of exhaustion. 

Metal Pin. Ranged Weapon Attack: +9 to hit, range 10/50 ft., 2 targets. Hit: 13 (2d4 + 9) piercing damage and the target must make a DC 16 Constitution saving throw (the target makes it with disadvantage if they're surprised) taking 18 (3d12) poison damage and are poisoned. On a success the damage is halved and the target is not poisoned, but they are stunned.

Coil of Pain and Death. If a creature enters the Ambush Vipers space, that creature makes a DC 16 Constitution saving throw with disadvantage taking 25 (5d10) poison damage and suffer 5 levels of exhaustion. On a success the creature takes half damage and only suffers 3 levels of exhaustion.