You are a doppleganger.  You give changelings a bad name.

Changeling

You are a Changeling of some kind, which enables you to polymorph yourself into other races or creatures, of your size, that you have seen.

Polymorphing changes the shape of your body and may affect your movement speeds, however it does not alter your hit points, attributes, skills, or abilities, except that you may not have access to some abilities while in other forms.  For example you may not be able to interact with certain objects if you do not have hands.  Any equipment you are wearing or carrying isn't transformed, and you revert to your true form if you fall unconscious or if you die.

You must be able to perform a type of movement to use it in your changed form, but if you can then you have your full movement speed if that movement type is natural to your changed form.  You can polymorph a number of times per short rest equal to your Charisma modifier and can hold a particular form as long as you are conscious.

You also have darkvision  up to 60 feet.

Doppelganger

At level 6 you are now a mature Doppelganger.  When you polymorph yourself, you can take the appearance of a specific individual that you have met, including one size category larger or smaller than your natural form.  You are also now able to use two Sorcery Points to recharge all your usages of your polymorph ability, and you do not revert to your natural form when you fall unconscious, but will if you die.

You are Immune to charm, and you have a Body Slam attack for 1d6 that you can use in any form you take.

Regardless of the appearance of your true form, you are now considered monstrous for purposes of detection and spell targeting.

Subversive

At level 14 you may choose a style of changeling abilities.

Ambusher

When you surprise an enemy you have advantage on attack rolls and your surprised target takes an extra 1d6 damage in the first round of combat.  You are also able to attack twice as your action.

Infiltrator

You can Read Thoughts at will, similar to the spell effect Detect Thoughts, you can read the surface thoughts of a single target continuously without detection.  While reading the target's mind, you have advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.

Your hearing and smell are enhanced giving you advantage on Wisdom (Perception) checks involving either of these senses.

Master Shapechanger

You have learned to take the shape of inanimate objects, and while in this form and remaining motionless you are indistinguishable from the real thing. You are also able to polymorph into mist in the shape of a humanoid. While in the Mist Form you are immune to physical attacks.