Ancient Doppelganger

Perfect Doppelganger.  The doppelganger can assume the form of a creature trapped within its pendant. While in that form, it gains full access to the creature’s class features, spells, and proficiencies. It uses its own hit points, saving throws, and passive features but otherwise functions as the impersonated creature.

Ancient Shapechanger. As a bonus action, the doppelganger magically transforms into a Small or Medium humanoid it has seen or trapped with its pendant, or back into its true form. It retains its statistics, except for its size and appearance. If the doppelganger spends one minute making the change, it can form the equivalent of any non-magical equipment carried by a creature trapped within its pendant.  This will not change the AC of the doppelganger, but can be used to create mundane weapons.  Otherwise, its equipment does not change.

Pendant of Identity Binding. The doppelganger wears a cursed amulet that imprisons a humanoid's body and soul in a demiplane. As an action (Recharge 6), the doppelganger targets one humanoid it can see within 30 feet. The target must make a DC16 Charisma saving throw.  If the target is charmed, frightened, unconscious, or surprised, they have disadvantage on the save.  On a failed save, the target is immediately transported to the pendant’s internal demiplane, incapacitated and unreachable unless the pendant is destroyed (AC 18, 25 HP, immune to necrotic, poison, psychic, and nonmagical damage). 

Multiattack. The doppelganger makes two Soul Piercer attacks, or casts a spell and makes one melee attack.

Soul Piercer. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 2d6+3 psychic damage. The target must succeed on a DC 14 Wisdom saving throw or perceive all creatures as hostile until the end of its next turn.

Mind Warp (Recharge 5–6). The doppelganger targets up to three creatures it can see within 30 feet. Each must make a DC 14 Intelligence saving throw, taking 4d10 psychic damage on a failed save and becoming stunned until the end of their next turn. On a success, a creature takes half damage and isn’t stunned.

Spellcasting.  The doppelganger uses Charisma as its spellcasting ability (spell save DC 14, +6 to hit with spell attacks). It does not require material components.
At Will: Thaumaturgy, Minor Illusion, Mage Hand
3/Day Each: Phantasmal Force, Mirror Image, Disguise Self, Detect Thoughts
2/Day Each: Phantasmal Killer, Counterspell
1/Day each: Synaptic Static