Atharina
The gist: A medusa with minor sorcerous powers that hides in plain sight in the high society of a decently sized urban centre.
Short background pre-Ravenloft: Atherina was living with her blind slaves (grimlock-types and people that have chosen blindness and servitude in order to avoid petrification) and guards in a different world. She was enjoying comforts, luxuries and wealth. The sources of those was often tribute from nearby settlements to leave them in peace, the occasional payment by evil people to use her as a mercenary or by downright robbery.
Atharina had a passing talent in arcane magic that she didn't develop much but she was interested in spells and magic magic items that would allow her to merge with the human society she longed to belong to and be accepted to (but without ending her evil ways - she wanted the luxuries, to be accepted and to instil fear to her enemies).
As her powers and infamy grew, she drew the attention of monster hunters. Even the ones she defeated, were creating chaos in her underground palace; blind servants have difficulty washing blood from the walls or repairing paintings.
And then, there was the ones she did not defeat. After several parties failed, one succeeded. Atharina barely managed to escape her lair with a handful of (expensive) gems and jewels, just one loyal slave and her magic mask. All her art, statues, dresses, books and poems were left behind as she fled like a monster. She and her servant entered a bunk of mists... and she came to Ravenloft.
Long term goals: Atherina, chased in her world and losing her palace, comforts and luxuries wants to be wealthy, comfortable and not be hunted. She still wants admiration and respect whether that respect is for her status or because she can turn people to stone.
Her magic and mask helps her to blend in.
The Mask (name pending): Atherina, shortly before exiled from her world has made a very special, elegant mask made by silver wire.
While wearing this mask, Atherina benefits from a "disguise self" spell that hides her appearance, making her appear as a tall human woman with tan complexion and thick braids or dreadlocks. Unlike the spell, the mask disguises the wearer (Atherina or other) to appear human of the same gender, built and characteristics close to the original. For example if the mask fell into the hands of a halfling, the halfling would appear as a human child wearing a mask. The mask allows minor alterations of that specific form, for example the wearer can choose her illusionary appearance to appear older, or with specific make-up, to add a tattoo, different color eyes or lips or minor cosmetic changes but in general, the illusionary image won't be drastically different.
The second, and more interesting property of that mask is that it nullifies the gaze attacks of the user. A vampire that is attuned to the mask will lose the charm gaze while wearing it and Atherina cannot turn people to stone while wearing it. She made the mask with the property on purpose as in my world(s), a medusa cannot "turn off" her gaze attack. It is not a bug, it's a feature.
When asked about the mask (that since it is a wire masks doesn't hide the illusionary features behind it), she says it is a cultural thing or a family tradition, or makes light of it asking her interlocutors if they think her "Face jewellery" needs an update.
Atherina's stats:
Atherina's stats are not impressive and they are similar to a 5th edition medusa with the following changes:
strength 9 instead of 10, intelligence 13 instead of 12 and she is also trained in arcana.
She also has less-than-average hit points. She still has 16 con and 17 hit dice, but she has 112 hp instead of 127 hp.
She does have a very nice longbow (white wood, decorated with brass wire and engravings - not magical, expensive but fully functional although heavier than a normal longbow) but she doesn't carry it with her except when she expects trouble.
Her CR is still 6, because Atherina can use the Shield spell to gain a lot of AC.
Atherina's magic:
Atherina is a 4th level sorcerer. The DCs to resist her spell is 13. She knows the following spells:
cantrips: Mage hand, mending, light, minor illusion, mold earth*
1st: Disguise self, shield
2nd: Alter self, enhance ability, suggestion
*I have nerfed mold earth to not be the ultimate excavation tool.
Atherina uses her magic to mostly cast enhance ability on herself. She casts it often to gain an advantage on negotiations, deception or even during research.
She would use a suggestion spell if needed, but she prefers to use natural persuasion as she is concerned that using suggestion too freely would lead back to her.
She rarely participates to any kind of negotiation or important social function without her charisma enchanced.
Metamagic: Subtle spell. Atherina uses her 4 spell points when needed to cast "enhance ability" unnoticed, in order to boost her already decent deception skill or her insight.
Atherina does not need her disguise self spell any more but she does need her alter self spell. While she avoids physical contact (as it can trigger someone disbelieving the illusion and seeing a monster in a dress) there are times where she can't completely avoid it. In those times, she uses alter-self to actually change her appearance for real; using the option to "lose" her snake attacks and natural armor. She doesn't like doing that as she fully embraces her monstrous nature. She wants to be accepted by humans as a superior, not become human.
The only occasions she now uses her disguise self spell is to over-write her mask's illusionary appearance to appear as someone very different or (rarely) to erase the image of the mask from the illusion.
Relationships: Atherina has partnered up with another villain (a disgraced human merchant seeking revenge) that I will present a different time. Their partnership started out of mutual benefit, it turned to trust and loyalty and she now has romantic feelings for the human. She is still an evil, selfish creature with a black heart, but she does have feelings for this man despite that.
She still has her loyal blind servant (a now old human that was born blind) and has acquired the services of a prim young rogue that acts as valet, informant and spy, a young man with low morals that has no idea he works for a monster and tries to seduce her, which she finds amusing. For the time. Once he annoys her enough or if she suspects him to be stealing from her despite his generous salary, he will be turned to stone. Thanks to her (magically boosted) business endeavors (and illegal activities), Atharina can now afford a bigger staff of servants that she hires on a temporal basis, for 2-3 years (she lets them go, she doesn't kill them). They are not aware of her nature and most don't suspect her illegal activities.
She does have a few other business contacts (and underworld contacts), some of which are aware that she is more than she seems but not what she is.