"Mirror mirror, on the wall - who shall live and who shall fall?"

Greek mythology is not always kind. If you look up the story behind Medusa, Stheno, and Euryale and what Poseidon does to them in a sacred temple - and then what their goddess does to them as a 'reward' - you should be upset. It is no wonder that Gary Gygax gave them 'lawful evil' as an alignment, they have very good reason to be upset with their heritage.

That said, most games do not give these girls a fair chance. Medusas (medusae?) have wit and charm aplenty... and forever to get it right. What should a fair and fun encounter with one of these women be like?

Medusa in Game - How They Look & What They Have

In some ways the 5th edition medusa keeps up with most traditions: a female (check), has snake hair (check), the snakes have some poison (check) and her gaze has petrification for those who fail to avert their eyes / saving throw (check). The mythology has been radically changed (i.e. all rape removed / now family friendly) and replaced with this:

"Immortal Splendor. Men and women who desire eternal youth, beauty, and adoration might pray to malicious gods, beg dragons for ancient magic, or seek out powerful archmages to fulfill their wishes. Others make sacrifices to demon lords or archdevils, offering all in exchange for this gift, oblivious to the curse that accompanies it. Those who strike such bargains gain physical beauty, restored youth, immortality, and the adoration of all who behold them, granting them the influence and power they so desire. However, after years of the living like a demigod among mortals, the price for their vanity and hubris is exacted, and they are forever transformed into medusas."

Monster Manual page 144

Apparently the long term consequence of asking powerful beings for sexy immortality: they transform you into a medusa. Is there any example of a spell or monster or stat-block or writeup of an actual dragon, god or demon that actually does this? Well, no. But let's just go with it, shall we?

A medusa has been dealt some really solid cards, all of which can be flipped with the right turn of the table.

Giving This 'Monster' A Fighting Chance - Fairly & Genuinely

Let us look at what she has: some good abilities, poison and at-will petrification. What would she do?

• She would get armour that would take the MOST advantage of her dexterity. This is breastplate (AC 14 & two points of dex) if she wants to make use of her stealth and half-plate (15 + 2 dex) if she is using non-stealth tactics. She would have a shield (+2) she could use when not using her longbow. If she can find the items (or the pattern-formulas), she would probably enchant all her tools to +1. Her armour class is 18 at worst and 21 with some simple +1 enchantments.
• She has plenty of good / free poison from her 'hair'. Use your head, girl! She would apply this 4d6 poisons to her swords and arrows. Also, all traps she set would include her poisons as well.
• Anyone who shows up in her lair-dungeon has either wandered in (uninvited) or is there to kill her (unwanted) or both. She will set up a slaughterhouse of lethal traps in her spare time (which she has lots of). If a beloved guest does show up, she could leave a bell-pull-rope-chain-chime thingy in the main foyer for them to announce their arrival. Otherwise, she is smart and wise enough to prepare all the defences she can.
• Her gaze-curse functions on any who: 1. see a real-direct or virtual-mirror image of her eyes; and, 2. are within 30' of her. She will have mirrors that bounce her image everywhere: up, down, around corners or anywhere.
• Her targets only need to see her eyes. She can use TOTAL cover, peering with her eyes through a tiny glass window. She would hide the rest of her body / not let on that she is a medusa. She would move her previous targets and victims and any other clue she was a 'medusa that turns folks to stone'. She would put down false clues that some other monster lives here now, like dragon claw marks or goblin equipment.
• Surprising her unwanted guests works really well for her. Example: target attackers with poison-trap attacks (like spring-darts or a falling-hooked net) - when targets search for yet more traps she rewards them with a tiny mirror that shows just her eyes. In such surprise situations she would be within 30' but behind a wall / 100% cover / only able to see the 'profile' of the curved-convex mirror (and not able to see her reflection in it).
• Her lair would require vision to survive. Narrow paths over rapids or cliffs, sharp objects sticking out of walls: easy to avoid as long as you are looking. She could peek-a-boo such invaders from any safe vantage mirror.

• With her charm and powerful supernatural contacts, she could easily be a warlock. Her high charisma and natural magical ability lends itself well to sorcerer and bard. Given the opportunity for study ('infinite time + endless solitude') she may also pick up wizardry. She has had plenty of contact with godlike creatures - she might worship any number of vengeful, bitter and angry gods. Sum up: she could have spells aplenty.
• Medusae are 'lawful evil' - they could have contacts galore. They may serve dragons, hags, more trustworthy drow and many other long-lived, law-abiding creatures. These contacts give medusae resources.
• Medusa could make ideal hags (assuming they 'hate beauty' by this point). Note: in lore, many hags are blind. If her two coven-sisters had their vision in special blind-senses, hag-eyes, familiars (especially blindsight with spiders) and certain spells. Gazing at medusae at 30'+ is perfectly safe.

Stepping It Up A Notch

• Maze of polished steel mirrors and glass - with cloth-cloaked statues that look like medusas: lair-invaders can see medusa-images everywhere. Which one is her? Such a maze would also be filled with poison traps that force people to keep an eye out. The medusa would probably be one level 'up' or 'down' so as to safely get within 30' and send her gaze down when players are ready. Make some of the glass easily shatterable by players - and the shards are all covered in poison.
• Have falling glass-mirror shard traps coated in her 4d6 poison covering an area.
• Enchant a special 'cursed' mirror-shield enchanted (common magic item) to be indestructible. The curse? Wielder of this shield cannot see their own reflection.
• Give her vampirism. She has no reflection. This reduces her mirror tricks but it makes her gaze weapon far harder to work against. This also increases her charisma, making the petrification save DC that much higher.
• If the medusa is legally blind her gaze still works. Extra points for giving her blindsight.
• She may have alchemy skills and use her poison as a magical / alchemical component. She may make sleep poisons like the Drow, healing potions and other poisons from the 5e list.
• Underwater-gaze situations are unexpected and offers her a lot of ranged defence. She could set this up with treasure in the water, she could be behind bars / portcullis with 100% cover ('iron wall with glass window'). She doesn't even have to be underwater herself: she could be looking through a water-sealed door with a glass visor.
• Blind minions like Grimlocks would gladly serve her. Also, non-corporeal undead and golems are immune to her gaze and may make ideal minions. Ghosts make excellent spies and will easily possess humanoids (especially 'blind' ones hiding their gaze / safety of darkness).
• Pathways where pedestrians MUST look (jumping stone to stone over rapids).
• Permanent illusions of medusae works well - along with the 'real' cloaked statues with moving hoodie such as is provided by the common magic item Cloak of Billowing. Some illusions could be walls that the medusa can
• Treasures: Medusa used to be rich / powerful before the fall, they have had a lot of time to accumulate or even craft wealth-goods and tend to have a storage of long-lasting heroes from down through the ages.

Adventures With Medusa(e): Stories Set In Stone

Creepy shadows: She has an invasion of undead shadows. Not only are they immune to her poison-petrification, she does not have much strength to survive the onslaught. Three unlucky hits and she is dead - forever! She is willing to pay handsomely in treasure and magic items.

Grimlocks cannot spell / want spells: A small group of well-dressed grimlocks explain their conundrum: "Master need spell. No, she not join 'guild'. Deal like so: you get spell, make copy. You send spell, she make copy. It all down here - see this?" They are willing to trade spell for spell so she can complete her wizardly book. Other wizards are interested - though some seek adventuring groups that would get all her treasure & wizard books lump sum.

Demons Want Help: Someone has been summoning many, many fiends and they don't come back. Some smart daemons are asking adventurers to arrange for the release of their kind. They will also pay you to keep various demons and devils where they are. Reward is with sentient magic items made of soul-material, re-forged obolos. "This talking Carpet of Flying is amazing - but who can trust it?"

Dragon Of Your Dreams: A metalic dragon haș been reaching out to adventurers with Dreams. She says that she has been petrified but can somehow still cast this one spell. She has been dreaming around for a while now - she sees that there are massive undead armies amassing and a mostly-good kingdom would soon be wiped out. This dragon asks that the adventuring party recruit this stoner-girl at all costs / post haste. She warns adventurers: this medusa has many tricks up her sleeve - but that is exactly why we need her services. Good luck!

Darkness Strikes: A medusa seems to have left her lair and has decided to turn an entire town to stone. She somehow knows where everyone is and is petrifying everyone! What does she want? Can she be reasoned with? if not, can she be stopped?

Follow-up to 'Darkness Strikes' - 'Gullet Goop Goods For Grabs': An alchemist has a bunch of bulging bulimic basilisks - and formulae for making basilisk gullet bile into (copious amounts of) Stone To Flesh elixir! This is great news - and so close to the town being petrified. What a lucky coincidence. I wonder what they could possibly want for such a life-saving potion?

Follow-up to 'Gullet Goop Goods For Grabs' 'Body Snatchers & Soul Stealers': Many of the statues of people came back without souls. They were in coma state when they came out of petrification and otherwise in mint condition. A few dozen ghosts showed up and were willing to help: they keep the bodies fed and cared for until their original owners return. What a lucky coincidence (again!), right?

Bad Girl Gone Good: Follow ups to above three quest-lines: One of the girls that was once the pride and joy of their parents disappeared for a few weeks - and she came back remembering nothing of her travels and was suddenly really horrible, albeit of the same memories and basic personality. It was as if she was changed somehow. Some time later she was petrified, then released from petrification and once she was all fleshy she was still missing her soul. Later, a old ghost of an old yet unjustly slain gay paladin took her over. Now he is an old gay man trapped in the body of a young soul-less girl that understands all the conspiracies:

• a medusa seeks to join a coven: she was willing to do anything to get membership!
• a night hag seeks to destroy a rival coven: she is willing to do anything to win!
• a green hag tried to have a changeling daughter: she is willing to do anything to get revenge!
• a town lost many of its most evil citizens - first petrified, then souls stolen by a night hag! They don't know what to do.
• an old gay paladin turned old genderless ghost turned hot pubescent teenager seeks revenge against... no one... because that just ain't right - but he / it / she has to do something! What can he-she do?
• one of the spirits was a 9th level druid - she can cast Reincarnation on their bodies and call back the souls, so long as it has been less than ten days. But the souls are horribly evil / they were the worst people in town. Should she bring them back?