Calliope Krait

 Magic Resistance. Calliope has advantage on saving throws and other magical effects. 

Spellcasting. Calliope is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). Calliope can cast disguise self and invisibility at will and has the following witch spells prepared:

Cantrips (at will): fire bolt, toll the dead, infestation, frostbite, shocking grasp

1st level (4 slots): shield, sleep, mage armor*, magic missile

2nd level (3 slots): silence, mirror image, misty step

3rd level (3 slots): counterspell, conjure barrage, lightning bolt

4th level (3 slots): banishment, fire shield, confusion

5th level (3 slots): cone of cold, conjure volley, contagion, temporal shunt

6th level (1 slot): eyebite

7th level (1 slot): power word pain

8th level (1 slot): maddening darkness*

9th level (1 slot): power word kill

*The medusa casts these spells on itself before combat.

Multiattack. The medusa makes either three melee attacks--one with its snake hair and two with its serpent blade.

Serpent Blade. Calliope creates a shadowy blade made out of pure magic. Melee Spell Attack +9 to hit, reach of 15ft,  one creature. Hit 26 (4d12) force damage. 

Snake Hair. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 15 (3d6 + 5) piercing damage plus 10 (3d6) poison damage.

Petrifying Gaze (Recharge 5-6). Calliope reveals her true form. Every creature that can see her within 30 feet must make a DC 17 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.

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.

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.

Serpent Blade. The medusa makes one attack with her serpent blade.

Animate Stone. The medusa animates 1 statue as animated armor.

Fey Step (Cost 2 Action). The medusa momentarily steps into the fey wild and teleports 30ft to an unoccupied space she can see and becomes invisible. 

Summon Demon (Cost 3 Action). The medusa summons a demon as if by the spell summon greater demon at will.