Bone Devil 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.

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.

Mimic ability. Once a self affecting ability or spell is seen by a doppelganger they are able to copy that ability or spell as long as they are Shapeshifted into the person the saw does the ability or spell

Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.

Devil's Sight. Magical darkness doesn't impede the devil's darkvision

Posion Combustion. Once at 0 hit points the doppelganger will explode shooting out posion and infecting all things within 10ft  (5d6) poison damage, and anyone in the range must succeed on a DC 14 Constitution saving throw or become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Multiattack. The doppelganger makes two melee attacks.

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit:8 (1d8 + 4) slashing 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.

Sting. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit:13 (2d8 + 4) piercing damage plus 17 (5d6) poison damage, and the target must succeed on a DC 14 Constitution saving throw or become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.