Baroness Cynthia (the night hag)

Legendary Resistance (2/Day). If Cynthia fails a saving throw, she can choose to succeed instead.

Devils Sight. Cynthia can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

Soul Devourer. When she reduces a creature to 0 hit points, Cynthia devourers the soul and can expend them to do one of the following; hag magic, dream eater and hurl through hell. Cynthia starts with ten souls and can have a maximum of fifty.

Innate Spellcasting. Cynthia innate spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). She can innately cast the following spells, requiring no material components:

At will: detect magic, magic missile, eldritch blast (repelling blast or grasp of hadar), invisibility,  

4/day each: plane shift (self only), ray of enfeeblement, sleep, darkness, dispel magic, hex, hold person, mirror image, hypnotic pattern, banishment, shadow of moil, eye bite, circle of death

1/day each: etherealness, gate, Maddening darkness, soul cage, scrying, finger of death, polymorph

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

Dark Ones Blessing. When Cynthia reduces a hostile creature to 0 hit points, she gains temporary 21 hit points.

Gift of the Dark One (1/day). If Cynthia's is reduced to 0 hit points but not killed outright, she magically drops to 1 hit point instead. 

Hag Magic. Cynthia manifests her twisted spells to create additional effects to sunder her enemies. She has the following metamagic options;

Quickened Spell (1/soul). When Cynthia casts a spell that has a casting time of 1 action, she can change the casting time to 1 bonus action for this casting.

Twinned Spell (1/soul). When Cynthia casts a spell that targets only one creature and doesn't have a range of self, she can target a second creature in range with the same spell. To be eligible, a spell must be incapable of targeting more than one creature at the spell's current level.

Heightened Spell (1/soul). When Cynthia casts a spell that forces a creature to make a saving throw to resist its effects, she can give one target of the spell disadvantage on its first saving throw made against the spell.

Multiattack. If Cynthia uses her claw or dream eater attack, she can cast a cantrip or a 1st level spell immediately after.

Claws (Hag Form Only). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 4) slashing damage.

Dream Eater (3/souls). Melee Weapon Attack: +8 to hit, reach 5 ft., one unconscious humanoid. Hit: The target takes 30 (6d10) psychic damage. If this damage reduces the target to 0 hit points, Cynthia devours the targets soul killing the creature. Cynthia then regains hit points equal to the targets maximum hit points.

Etherealness. Cynthia magically enters the Ethereal Plane from the Material Plane, or vice versa. To do so, Cynthia must have a heartstone in her possession.

Nightmare Haunting (1/Day). While on the Ethereal Plane, Cynthia magically touches a sleeping humanoid on the Material Plane. A protection from evil and good spell cast on the target prevents this contact, as does a magic circle. As long as the contact persists, the target has dreadful visions. If these visions last for at least 1 hour, the target gains no benefit from its rest, and its hit point maximum is reduced by 5 (1d10). If this effect reduces the target's hit point maximum to 0, the target dies, and if the target was evil, its soul is trapped in Cynthia's soul bag. The reduction to the target's hit point maximum lasts until removed by the greater restoration spell or similar magic.

Hurl Through Hell (1/day or expend 6 Souls). When Cynthia hits a creature with an attack, she can instantly transport the target through the lower planes. The creature disappears and hurtles through a nightmare landscape. At the end of Cynthia's 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 50 (10d10) psychic damage as it reels from its horrific experience.

Drifting Off. When Cynthia is hit with a spell and the target who cast the spell has a total of 20 hit points or less fall asleep (unconscious) until the end of Cynthia's next turn.

Tomb of Levistus (1/day). When Cynthia takes damage, she can entomb herself in thick dark ice, which melts away at the end of her next turn. Cynthia gains 170 temporary hit points, which take as much of the triggering damage as possible. Immediately after Cynthia takes the damage, she gains vulnerability to fire damage, her speed is reduced to 0, and she is incapacitated. These effects, including any remaining temporary hit points, all end when the ice melts.

Cynthia 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. Cynthia regains spent legendary actions at the start of its turn.

Cantrip. Cynthia casts a cantrip.

Sleep (Costs 2 Actions). Each creature with in 10 feet of Cynthia must make a DC 15 constitution saving throw. On a fail the creatures fall unconscious until the end of their next turn.

Reckless Attack (Costs 3 Actions). Cynthia makes six claw attacks against a creature with in range.