Sul Khatesh Assumes Control (Recharges after a Short or Long Rest). If Zandria would be reduced to 0 hit points, her current hit point total instead resets to 1 hit point, she recharges her 5th level spell slots, she regains any expended uses of Legendary Resistance, and gains 75 temporary hit points. Additionally, Zandria can now use the Consume Magic legendary action during the next hour, and the number of temporary hit points she has resets to 75 temporary hit points at the start of each of her turns. Award a party an additional 13,000 XP (26,000 XP total) for defeating Zandria after Sul Khatesh Assumes Control activates.

Disheartening Strike. When Zandria hits a creature with an attack using the Acheron Greatsword, she can fill the target with unsettling dread: the target has disadvantage on the next saving throw it makes before the end of her next turn. The creature ignores this effect if it’s immune to the frightened condition. She can use this property 3 times per day. 

(Eldritch) Smite. When Zandria hits a creature with a melee weapon attack, she can expend a 5th level spell slot to deal an extra (6d8) force damage to the target

Magic Resistance. Zandria has advantage on saving throws against spells and other magical effects.

Pact Magic. Zandria is an 18th-level spellcaster. Her spellcasting ability is Charisma(spell save DC 18, +10 to hit with spell attacks). Zandria has the following warlock spells prepared, and regains the spell slots on a short rest:

Cantrips (at will): chill touch, mage hand, prestidigitation, shocking grasp, toll the dead

5th level (4 slots): fireball, banishment, synaptic static, scrying, armor of agathys, counterspell, dispel magic, fire shield, hallow, banishing smite

Zandria can cast the following spells a number of times per day without expending a spell slot. 

1/day: mental prison, crown of stars *, maddening darkness, psychic scream

*Zandria casts these spells on herself before combat.

Multiattack. Zandria makes 2 melee weapon attacks with her Acheron Blade, four spell attacks with her fiendish blast, or one melee attack and two spell attacks.

Acheron Greatsword. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 19 (3d6 + 9) slashing damage.  

Fiendish Blast. Ranged Spell Attack: +10 to hit, range 120 ft., two targets. Hit: 11 (1d10+5) force damage.  

Hurl Through Hell (1/Day). When Zandria hits a creature with an attack, she can use this feature to instantly transport the target through the lower planes. At the end of her next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic damage. Once she uses this feature, she can't use it again until she finishes a long rest.

Assume Control. Zandria can assume control of While she assumes control of an adjusted, it has an additional 10 hit point health regeneration per turn and access to the Fiendish Blast Legendary action and gains one legendary action. Additionally, the Intelligence, Wisdom, and Charisma stats of the Adjusted are replaced by Zandria's

Crown of Stars. Since crown of stars is already cast before combat, she can shoot one of her stars as a bonus action.

Parry. Zandria adds 5 to its AC against one melee attack that would hit her. To do so, she must see the attacker and be wielding a melee weapon.

Zandria can take 2 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. Zandria regains spent legendary actions at the start of its turn. Fiendish Blast. Zandria makes two Fiendish Blast attacks

Attack. Zandria makes a weapon attack

Move. Zandria moves up to her speed without taking opportunity attacks.

When Zandria changes form, she gains another legendary action, for a total of 3. Additionally, she gains access to a new Legendary Action:

Consume Magic (Costs 2 Actions). Sul Khatesh targets a creature within 120 feet of her who is concentrating on a spell. The target must succeed on a DC 26 Constitution saving throw or its concentration is broken on the spell, and Sul Khatesh gains 5 temporary hit points per level of that spell.