Aleta, Gavarenth Incarnate

Mythic Trait: Incarnation of Gavarenth (Condition: Taking damage from Rubrum Cucullo or hitting 0 hit points). Aleta regains hit points until she reaches 100, or if she is above 100 hit points she regains no hit points. Aleta's human form begins to crack away as Gavarenth is truly awakened within her, causing her eyes to glow bright purple, scales to form across her body, and a pair of black wings to grow from her back. She gains a flight speed of 60 feet.
If Aleta's hit points are reduced to 0 by an attack from Rubrum Cucullo while Mythic, Gavarenth the Nocturnal is released from her body. Gavarenth the Nocturnal uses the stats of an Ancient Black Dragon, except it has all of the Legendary and Mythic actions of Aleta, Gavarenth Incarnate in addition to its original Legendary actions, can cast any spell Aleta, Gavarenth Incarnate could cast, and all instances of Acid Damage are replaced with Necrotic Damage.

Dual Mind. Aleta has advantage on Wisdom saving throws.

Sneak Attack. Once per turn, Aleta can deal an extra 2d6 damage to one creature Aleta hit with an attack with a finesse or ranged weapon if Aleta has advantage on the attack roll. Aleta doesn't need advantage on the attack roll if another enemy of the target is within 5 ft. of it, that enemy isn’t incapacitated, and she doesn't have disadvantage on the attack roll.

Cunning Action. Aleta can take a bonus action on each of her turns to take the Dash, Disengage, or Hide action.

Wails from the Grave [6/LR]. Immediately after Aleta deals her Sneak Attack damage to a creature on her turn, she can target a second creature within 30 ft. of it. The second creature takes 1d6 psychic damage.

Master of Hexes. When the creature cursed by Aleta's Hexblade’s Curse dies, she can apply the curse to a different creature she can see within 30 ft. of her, provided she isn’t incapacitated, but Aleta doesn’t regain HP from the death of the previously cursed creature.

Hexblade's Curse. Once per short rest, as a bonus action, Aleta can choose one creature she can see within 30 ft. to curse it for 1 minute (or until the target dies, Aleta dies, or Aleta is incapacitated). Against the cursed target, Aleta gains a +6 bonus to damage rolls, scores a critical hit on a roll of 19 or 20, and she regains 22 HP if it dies.

Eldritch Invocations.

Accursed Specter. When Aleta slays a humanoid, she can cause its spirit to rise as a specter that gains +8 temp HP. Roll initiative for the specter, which has its own turns. It obeys her verbal commands, gains a +5 bonus to its attack rolls, and remains until the end of her next long rest.

Alert. Aleta gains a +5 bonus to initiative, can't be surprised while conscious, and other creatures don’t gain advantage on attack rolls against her as a result of being unseen by her.

Sentinel. When Aleta hits a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn. Creatures provoke opportunity attacks from her even if they take the Disengage action. When a creature within 5 ft. of Aleta makes an attack against a target other than her, Aleta can use her reaction to make a melee weapon attack against the attacking creature.

War Caster. Aleta has advantage on CON saving throws made to maintain her concentration on a spell when she takes damage. Aleta can perform the somatic components of spells even when she has weapons or a shield in one or both hands. Aleta can use her reaction to make an opportunity attack to cast a spell that has a casting time of 1 action and targets only that creature.

Spellcasting. Aleta is a 17th level spellcaster. Her spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). Aleta has following Warlock spells prepared:
Cantrips (at will): Eldritch Blast, Mage Hand, Prestidigitation, Thunderclap
5th level (4 slots): Banishment, Blight, Blink, Blur, Charm Person, Counterspell, Darkness, Dispel Magic, Expeditious Retreat, Hellish Rebuke, Hold Monster, Scrying, Spirit Shroud, Vampiric Touch
6th level (1/LR): Circle of Death
7th level (1/LR): Finger of Death
8th level (1/LR): Feeblemind
9th level (1/LR): Mass Death

Multiattack. Aleta makes 3 attacks, using any combination of her shortsword, Rubrum Cucullo, and Eldritch Blast, however she may only use Eldritch Blast for one of the attacks.

Shortsword. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 9 (1d6 + 4) piercing damage.

Rubrum Cucullo. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 9 (1d4 + 7) piercing and 4 (1d6) cold damage.

Armor of Hexes. If the target cursed by Aleta's Hexblade’s Curse hits her with an attack roll, she can use her reaction to roll a d6. On a 4 or higher, the attack instead misses Aleta, regardless of its roll.

Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary Actions are regained at the start of each turn.

Cantrip. Aleta casts a cantrip.

Shadow of the Calamity (Costs 2 Actions). Aleta summons a Spectre in an unoccupied space within 30 feet. This Spectre is allied with her and takes its turn immediately after Aleta's. Aleta can maintain up to 3 Spectres at a time, with the oldest ones vanishing if more are summoned.

Visions of the Calamity (Costs 3 Actions). Each non-Survivor of the Calamity that can see Aleta must make a DC 18 Wisdom saving throw, taking 21 (6d6) psychic damage on a failed save, or half as much damage on a successful one.

Can take 3 Mythic Actions, choosing from the options below. Only one Mythic Action can be used at a time. Spent mythic Actions are regained at the start of each turn.

Scales of Gavarenth (Costs 1 Action). When Aleta would take damage from a magical damage source, she can instead gain immunity to that damage type for the duration of the attack and regain hit points equal to half the damage that she would have taken. Aleta may only perform this Mythic Action once per magical damage source per combat.

Blood of Gavarenth (Costs 2 Actions). For the duration of this round, when Aleta scores a critical hit against a target, that target takes an additional 4d6 necrotic damage and Aleta gains temporary hit points equal to half the necrotic damage dealt.

Claws of Gavarenth (Costs 1 Action). For the duration of this round, all of Aleta's melee attacks have a minimum range of 30 feet and deal an additional 1d6 force damage as massive shadowy claws form from her weapons to strike foes.

Summon the Calamity (Costs 3 Actions). If Aleta has taken no Mythic Actions for at least 3 consecutive rounds, is wielding Rubrum Cucullo, and is within her spell circle, she can cast Mass Death at a speed of 1 Action.