Changeling Assassin

Change Appearance. The changeling can use its action to polymorph into a Medium humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.

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

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

Sneak Attack. Once per turn, the assassin deals an extra 21 (6d6) 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 the assassin doesn't have disadvantage on the attack roll.

Innate Spellcasting. The changeling's spellcasting ability is Charisma (spell save DC 14). The changeling assassin can innately cast the following spells, requiring no material components:

2/day: sleep, invisibility (self only)

Special Equipment. Each Changeling Assassin carries a Band of Loyalty, and two beads each from Necklace of Fireballs and Bead of Force split evenly between the 3-4 of them.

Multiattack. The assassin makes up to two weapon attacks. 

Short Sword. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 8 (1d6 + 5)

Hand crossbow. Ranged Weapon Attack: +9 to hit, range 30/120 ft., one target. Hit: 8 (1d6 + 5) piercing damage.

Concealed Blade. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 8 (1d6 + 5) piercing damage plus 10 (3d6) poison damage, and the target must succeed a DC 16 Constitution Saving throw or become Poisoned for 10 minutes. Creatures are not aware of the concealed blade unless they have a passive perception of 15 or higher or pass a DC 15 perception check. If the target doesn't see the blade, the assassin has advantage on the first attack with it. (The blade is attached directly to the changeling's arm, attempting to remove the blade will destroy the contraption itself and all of its effects.)