Changeling Assassin

Shapechanger. The changeling can use its action to polymorph into a Small or 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 changeling has advantage on attack rolls against any creature that hasn't taken a turn yet. Any hit the changeling scores against a surprised creature is a critical hit.

Sneak Attack (1/Turn). The changeling deals an extra 14 (4d6) 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 changeling that isn't incapacitated and the changeling doesn't have disadvantage on the attack roll.

Evasion. If the changeling is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage on a success and half damage on a failure.

Multiattack. The changeling makes three attacks: two with its hand axe or two with its hand crossbow.

Hand Axe. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 4) piercing damage.

Hand Crossbow. Ranged Weapon Attack: +6 to hit, range 30/120 ft., one target. Hit: 7 (1d6 + 4) piercing damage.

Second Wind (1/Short Rest). The changeling regains 13 (1d10 + 3) hit points.

Cunning Action. The changeling can take a bonus action to Dash, Disengage, or Hide.

Infiltrator's Insight. The changeling makes a Wisdom (Insight) check contested by the Charisma (Deception) check of a creature it can see. If the changeling succeeds, it has advantage on attack rolls and ability checks against the creature until the end of its next turn.

Uncanny Dodge. When an attacker the changeling can see hits it with an attack, it can use its reaction to halve the attack's damage against it.