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.

Touch of the Abyss: The Doppelganger will release a plume of caustic gas upon its death and it can turn Invisible at will.

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

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.

Multiattack. The doppelganger makes two melee attacks.

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

Reality Warp. The Doppelganger can cast powerful Illusion that effect the target in a verity of ways depending on the information collected from their thoughts sending them into a dream-like state where they are unable to act against the Doppelganger unless the pass a DC 18 Wisdom saving throw. If the target has lost loved ones the Doppelganger spins an Illusion of said loved ones being alive and well as reality shifts to the loved ones house. If the target lusts for women/men the Doppelganger will become the perfect partner of the target and reality shift to a bedroom for some fun time. If the target seeks knowledge the Doppelganger will take the form of a knowledgeable scholar and reality will shift to a massive and beautiful Library. If the target is religious it will take the form of the deity they worship and reality will shift to a ornate and spacious temple. The Doppelganger will choice its approach according to the information it gathers while stalking its prey.

Disappearing Act: The Doppelganger turns invisible until the end of combat or until the attack or cause a saving throw.

Fade Away: If damage by an attack the can turn invisible until the end of combat or until they attack.