Ancient Medusa

Blood of the Serpents. A creature that hits the medusa with a melee attack that deals piercing or slashing damage and is within 5 feet of it takes 5 (1d10) poison damage. Additionally, a swarm of poisonous snakes appears in an unoccupied space within 5 feet of the medusa, and acts as an ally of the medusa.

Legendary Resistance (3/Day). If the medusa fails a saving throw, it can choose to succeed instead.

Petrifying Gaze. When a creature that can see the medusa's eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 18 Constitution saving throw if the medusa isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic. 

   Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.

 If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.

Spellcasting. The medusa is a 12th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following sorcerer spells prepared:

Cantrips (at will): chill touch, minor illusion, mending, poison spray, shape water

1st level (4 slots): shield, thunderwave

2nd level (3 slots): hold person, misty step

3rd level (3 slots): counterspell, fireball

4th level (3 slots): charm monster, dimension door, vitriolic sphere

5th level (2 slots): cone of cold, enervation

6th level (1 slot): globe of invulnerability

Sorcery Points. The medusa has 12 sorcery points. It can spend 1 or more sorcery points to gain one of the following benefits:

 Heightened Spell: When the medusa casts a spell that forces a creature to make a saving throw to resist the spell's effects, the medusa can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw against the spell.

 Quickened Spell: When the medusa casts a spell that has a casting time of 1 action, the medusa can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.

  Subtle Spell: When the medusa casts a spell, it can spend l sorcery point to cast the spell without any somatic or verbal components.

Multiattack. The medusa makes either three melee attacks - one with its snake hair, one with its scimitar, and one to constrict - or three ranged attacks with its longbow.

Snake Hair. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 6 (1d4 + 4) piercing damage plus 21 (6d6) poison damage.

Scimitar. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 11 (2d6 + 4) slashing damage plus 10 (3d6) poison damage.

Constrict. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, the target is restrained, and the medusa can't constrict another target.

Longbow. Ranged Weapon Attack: +10 to hit, range 60 ft., one target. Hit: 13 (2d8 + 4) piercing damage plus 10 (3d6) poison damage.

The medusa can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The medusa regains spent legendary actions at the start of its turn.

Attack. The medusa makes a scimitar or longbow attack or uses Constrict.

Force Petrifying Gaze (Costs 2 Actions). The medusa forces a creature grappled and restrained by it to meet its gaze. The target must succeed on a DC 18 Constitution saving throw or become petrified (see Petrifying Gaze).

Cast a Spell (Costs 2 Actions). The medusa casts a spell from its list of prepared spells, using a spell slot as normal.