Stone Cursed. When Aasia's hit points are reduced below 200, her stone skin shatters, revealing a crystalline creature held together by vein-like tendrils of infestation. Aasifa drops concentration on any spell she is currently concentrating on and is placed under the effect of the Investiture of Shadow spell.
When in her crystalline form, she loses her spellcasting and dark one's blessing traits, her vulnerability to bludgeoning damage, and her Wyvern's Breath attack.
Aasifa's walking speed becomes 40 and she gains a flying speed of 25.
Aasifa's Armor Class is reduced by 2, and she gains a vulnerability to fire and thunder damage.
In this form, if Aasifa is targeted by an effect that would deal necrotic or poison damage, she takes no damage and instead regains a number of hit points equal to half the damage dealt.

Spellcasting. Aasifa is a 14th-level eldritch spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). Aasifa has following warlock spells prepared:
Cantrips (at will): Chill Touch, Eldritch Blast, Infestation, Toll the Dead
5th level (3 slots): Blight, Blindness/Deafness, Compulsion, Counterspell, Darkness, Dimension Door, Dispel Magic, Hex, Hold Person, Misty Step, Negative Energy Flood, Shatter, Slow, Vampiric Touch
Arcanum (1/day): Investiture of Shadow, Finger of Death

Dark One's Blessing. When Aasifa reduces a creature to 0 hit points, she gains 19 hit points.

Virulent Cloak. A creature that starts its turn within 5 ft of Aasifa takes 5 necrotic damage. This damage doubles if Aasifa has less than 200 hit points remaining.

Maddening Hex. As a bonus action, Aasifa can create a psychic disturbance around a creature affected by her hex spell if it is within 30 feet of her, dealing 5 psychic damage to that creature and any number of creatures within 5 ft of it.

Hurl Through the Abyss. Once per day, when Aasifa hits a creature with an attack, she can teleport them to a demiplane of nightmares. If the creature is not a fiend, they take 10d10 psychic damage.

Frenzy Staff. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage, or 12 (2d8 + 3) bludgeoning damage if used with two hands. The target must also make a DC 15 Constitution saving throw. On a failure, the target takes 11 (2d10) necrotic damage and has disadvantage on all ability checks and saving throws using Strength or Constitution for 1 hour. A remove curse spell or similar effect ends this effect early. On a success, the target takes half as much damage and suffers no additional penalties.

Wyvern's Breath (Recharge 6). Aasifa chooses up to three points within 30 feet, lobbing an orb of necrotic energy at each of those points. Each creature within 5 feet of one of these points must make a DC 15 Dexterity saving throw, taking 7 (2d6) necrotic damage on a failure and half as much on a success.