Petrifying Gaze -  (Recharge 9-10 on initiative 20 Phase 2-4 only). As an action, Kallista may attempt to petrify everyone in a 15 foot cone in front of her. Each creature in the cone make a DC 13 constitution saving throw, taking 2d6 necrotic damage on a failed save or half damage on a successful save. A target that is intentionally averting their gaze automatically succeeds on this save. Each target that fails its save begins to become petrified and is restrained. At the end of the creatures next turn, it must make the saving throw again. On a failed save the target is petrified. 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.

Spellcasting. The Kallista is an 5th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +6 to hit with spell attacks). The Kallista can cast disguise self at will and has the following wizard spells prepared:

Cantrips (at will): poison spray, toll the dead, mage hand, sacred flame (necrotic damage).

1st level (4 slots): ray of sickness, ice knife (Reskinned for poison), arms of hadar

2nd level (3 slots):  acid arrow, misty step, Shatter

Turns: Kallista gains a second turn in phase 2 and a third turn in phase 4. These turns are taken after another player (The DM chooses which turn she goes).

Spell Slots: Kallista recovers one spell slot upon reaching phase 2 and phase 4.

Multiattack. Kallista makes either two melee attacks with her sword or longbow.

Sword (Phase 1). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 2) piercing damage plus 1 poison damage or 1 necrotic damage.

Sword (Phase 2). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 2) piercing damage plus 3 (1d4+1) poison damage or 3 (1d4+1) necrotic damage.

GreatSword (Phase 3). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) slashing damage plus 3 (1d6+1) poison damage or 3 (1d6+1) necrotic damage.

Sword+ (Phase 4). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 2) piercing damage plus 4 (1d6+2) poison damage or 4 (1d6+2) necrotic damage.

Longbow. Ranged Weapon Attack: +6 to hit, reach 150/600 ft., one target. Hit: 7 (1d8+3) piercing damage and 3 (1d6) poison damage.

Legendary Resistance (1/Day). If Kallista fails a saving through, she may choose to succeed instead.

Upon reaching 180 HP, Kallista saves against any ongoing magical effects (positive and negative). She also regains 1 spell slot and begins Phase 2. 

Upon reaching 80 HP, Kallista saves against any ongoing magical effects (positive and negative), gains the benefits of barkskin and begins Phase 3.

Upon reaching 35 HP, Kallista saves against any ongoing magical effects (positive and negative), gains the benefits of barkskin and begins Phase 4.