Crystal Assassin

Multiattack. As an action the Crystal Assassin can take one attack with its arm-blade and a second attack with its crossbow. 

 Toxin Utilization. The Crystal Assassin has a copper dial on the center of their chest which changes the type of poison being applied to the weaponry. There are three types of poisons;

Wyvern Poison. A creature subjected to this poison must make a DC 15 Constitution saving throw, taking 7d6 poison damage on a failed save, or half as much damage on a successful one.

Creeper's Sun. A creature subjected to this poison must succeed on a DC 18 Constitution saving throw or become petrified. On a successful save, the creature instead petrifies over the course of 1 week, losing 5 ft of movement speed per day until petrified. Another creature may attempt a DC 25 medicine check to make an antidote

Anti-magic Oil. This poison is a liquid that contains energies that inhibit magic. Whenever you hit with this poison on your weapon, your victim must make a DC 15 Constitution saving throw to avoid the oil being applied. If the target fails, the oil is transferred to them from your weapon and lasts for one minute. Each time they attempt to cast a spell, they must make a DC 15 Constitution saving throw. On a failure, the spell fails but any spell slot used to cast it is still spent. On a success, the oil dissipates and the spell is cast as normal.

Arm-blade. Melee Weapon Attack: +9 to hit, reach 5ft., one target. Hit: (2d4+7) Piercing Damage. 

Mounted Crossbow. Ranged Weapon Attack: +9 to hit, range 30/120, one target. Hit: (1d12+7) piercing damage 

Uncanny Dodge. When an attacker that it can see hits it with an attack, the assassin can use their reaction to halve the attack's damage.