Ancient 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.

Ambusher. The doppelganger has advantage on attack rolls against any creature it has surprised for 2 rounds.

Surprise Attack. If the doppelganger surprises a creature and hits it with an attack during the first 2 rounds of combat, the target takes an extra 27 (8d6) damage from the attack.

Multiattack. The doppelganger makes two melee attacks.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 12 (1d6 + 8) bludgeoning damage. Add 1d4 if enlarged or subtract 1d4 if shrunk.

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.

Evasion. Your instinctive agility lets you dodge out of the way of certain area effects, such as a blue dragon’s lightning breath or a fireball spell. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

Enlarge (Recharges after a Short or Long Rest). For 1 minute, the duergar magically increases in size, along with anything it is wearing or carrying. While enlarged, the duergar is Large, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the duergar lacks the room to become Large, it attains the maximum size possible in the space available.

Invisibility (Recharges after a Short or Long Rest). The duergar magically turns invisible until it attacks, casts a spell, or uses its Enlarge, or until its concentration is broken, up to 1 hour (as if concentrating on a spell). Any equipment the duergar wears or carries is invisible with it.

Parry +2 AC when an attack would hit which may cause it to miss instead. Once per round.