Shapechanger. The undead 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.

Ambusher. The undead doppelganger has advantage on attack rolls against any creature it has surprised.

Insight Fear. The undead doppelganger feeds off the fear of its victims. Whenever it attacks a creature affected by the frightened condition the target suffers an extra 8 (3d4) psychic damage.

Surprise Attack. If the undead 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.

Multiattack. The undead doppelganger makes two melee attacks or one bite attack.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage.

Bite.  Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (4d6 + 4) slashing damage. The target will also need to make a DC16 Strength saving throw or become grappled by the undead doppleganger.

Read Thoughts. The unead 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.