Brood Doppelganger

Shapechanger. The Brood doppelganger cannot use its action to polymorph into a Small or Medium humanoid like that of a Doppelganger. Instead, it has been designated a target to change into from its creator, by way of a blood transfusion. It cannot revert back into its true form until it drops to zero hit points, at which point the Brood Doppelganger reverts to its true form if it dies.. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed.

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

Surprise Attack. If the Brood 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.

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

Discern. The Brood doppelganger magically gets a taste of the surface thoughts of one creature within 10 feet of it. This sense is not as powerful as the "Read Thoughts" ability of a fully-developed Doppelganger. The Discern effect cannot penetrate barriers, and the target must remain in range of the doppelganger for 2 minutes before it can begin. 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.