Changeling

CAPGRAS REALITY
''Gripping a knife tightly and awkwardly in both hands, a woman in a white fancy dress speckled with blood runs through the halls of her massive family estate, hellbent on destroying every last inhabitant inside. She's convinced her family has been replaced by foul creatures that wish to gain her inheritance. She went down then into the burial vault where she knew her sister was hiding, tears streaming down her cheeks and one of her dainty hands clutching a crucifix around her neck. Unexpectedly, she heard her sister's voice behind her--not ten paces away as she began to plead and beg her to stop. The woman dropped the knife and embraced her sister, muttering apologies. And the sister grinned wide from ear to ear out of the woman's view knowing full well that inheritance was hers. This was not her sister.''

Biology
Changelings can assume the form of any medium-size humanoid creature, and have semisolid constantly mutating and shifting bodies. When in their original form, they appear as faceless, thin humanoids with pale skin and straight, wiry white hair or they can take a form of their own design.

Culture
Changelings often blend into the cultures of those around them, their peers often not knowing at all of their true nature.

Origin
Changelings were created alchemically by high-ranking Seelie court alchemists of the Feylands. Over many years of contemplation and experimentation (namely on infant Elves) they had finally achieved exactly what they were aiming for--a creature that can blend in perfectly with their their enemies' ranks and be perfect agents of reconnaissance.

Statistics 1
Source: Eberron: Rising from the Last War. You can find the text hosted here as well.

• Ability Score Increase. One ability score of your choice increases by 1.

• Change Appearance. As an action, you can change your appearance and your voice. You determine the specifics of the changes, including your coloration, hair length, and sex. You can also adjust your height and weight, but not so much that your size changes. You can make yourself appear as a member of another race, though none of your game statistics change.

You can't duplicate the appearance of a creature you've never seen, and you must adopt a form that has the same basic arrangement of limbs that you have. Your clothing and equipment aren't changed by this trait.

You stay in the new form until you use an action to revert to your true form or until you die.

Statistics 2
Source: Wayfinder's Guide to Eberron

• Ability Score Increase. Either your Dexterity or your Intelligence increases by 1 (your choice).

• Change Appearance. As an action, you can transform your appearance or revert to your natural form. You can’t duplicate the appearance of a creature you’ve never seen, and you revert to your natural form if you die.

You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change. You also can’t appear as a creature of a different size than you, and your basic shape stays the same; if you’re bipedal, you can’t use this trait to become quadrupedal, for instance. Your clothing and other equipment don’t change in appearance, size, or shape to match your new form, requiring you to keep a few extra outfits on hand to make the most compelling disguise possible.

Even to the most astute observers, your ruse is usually indiscernible. If you rouse suspicion, or if a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.

• Unsettling Visage. When a creature you can see makes an attack roll against you, you can use your reaction to impose disadvantage on the roll. You must use this feature before knowing whether the attack hits or misses.

Using this trait reveals your shapeshifting nature to any creature within 30 feet that can see you. Once you use this trait, you can’t use it again until you finish a short or long rest.

• Divergent Persona. You gain proficiency with one tool of your choice. Define a unique identity associated with that proficiency; establish the name, race, gender, age, and other details. While you are in the form of this persona, the related proficiency bonus is doubled for any ability check you make that uses that proficiency.