Advanced Doppelganger

Shapechanger. The doppelganger 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.

Improved Ambusher. During its first turn, the doppelganger has advantage on attack rolls against any creature that hasn't taken an action.

Surprise Attack. If the doppelganger surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack.

Cunning Action. On each of its turns, the doppelganger can use a bonus action to take the Dash, Disengage, or Hide action.

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

Sneak Attack. Once per turn, the doppelganger 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 doppelganger that isn’t incapacitated and the doppelganger doesn’t have disadvantage on the attack roll.

Multiattack. The doppelganger makes three melee attacks with either its slam or its shortsword. Or the doppelganger makes two ranged attacks with its hand crossbow.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage.

Shortsword. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.

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

Read Thoughts. The doppelganger magically reads the surface thoughts of one creature within 60 feet of it. The effect can penetrate barriers, but 3 feet of wood or dirt, 2 feet of stone, 2 inches of metal, or a thin sheet of lead blocks it. While the target is in range, the doppelganger can continue reading its thoughts, as long as the doppelganger's concentration isn't broken (as if concentrating on a spell). While reading the target's mind, the doppelganger has advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.

Uncanny Dodge. The doppelganger halves the damage that it takes from an attack that hits it. The doppelganger must be able to see the attacker.