Challenge 18 ish (the math comes out to an 18 but I would eye ball a 16 to be honest)
Baba Yaga, the demon hag of iron teeth Medium fiend, neutral evil
Armor Class 18 (natural armor) Hit Points 155 (20d8 + 60) Speed 30ft.
STR 18 (+4) DEX 15 (+2) CON 16 (+3)
INT 19 (+4) WIS 24 (+7) CHA 16 (+3)
Skills Arcana +8, Deception +7, Insight +11, Perception +11, Stealth +6
Damage Resistances cold, fire; bludgeoning, piercing, and slashing from non silvered weapons
Condition Immunities charmed, poison
Senses darkvision 120ft., passive Perception 21
Languages Abyssal, Common, Infernal, Primordial
Foretelling. Baba Yaga can see the future, so every morning she rolls 4d20, and records the results, as a non action at anytime she can place one of those dice instead of any D20 roll she or a target within 60ft makes, but it must be done before the roll, and can only be done once per round. Each d20 can only be used once.
Innate Spellcasting. The Yaga's innate spellcasting ability is Charisma (spell save DC 13, +5 to hit with spell attacks). She can innately cast the following spells, requiring no material components:
At will: detect magic, magic missile
2/day each: plane shift (self only), ray of enfeeblement, sleep
Spell casting. Baba Yaga counts as a 17th level warlock for casting spells, and wisdom is her prime stat spell save DC 19, +11 to hit with spell attacks).
Cantrips: eldritch blast(+3 damage per ray), prestidigitation, poison spray, thaumaturgy
Level 1 spells: burning hands, charm person, ferie fire, hellish rebuke, hex
Level 2 Spells: cloud of daggers, crown of madness, mirror image, scorching ray
Level 3 Spells: blink, counter spell, fly, hunger of hadar
Level 4 Spells: blight, dominate beast, fire shield
Level 5 spells (4 slots): scrying, seeming, hollow
Level 6 spells (1 slot): flesh to stone
Level 7 spells (1 slot): finger of death
Level 8 spells (1 slot): power word stun
Level 9 spells (1 slot): foresight
Magic Resistance. The hag has advantage on saving throws against spells and other magical effects.
Misty escape. Yaga can vanish in a puff of mist in response to harm. When she takes damage, she can use her reaction to turn invisible and teleport up to 60 feet to an unoccupied space she can see. she remains invisible until the start of her next turn or until she attacks or cast a spell. Once she use this feature, she can't use it again until she finishs a short or long rest.
Hurl through hell. (recharge 6) When yaga 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 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 50(10d10) psychic damage as it reels from its horrific experience.
ACTIONS
Claws (Hag Form Only). Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage.
Change Shape. The hag magically polymorphs into a Small or Medium female humanoid, or back into her true form. Her statistics are the same in each form. Any equipment she is wearing or carrying isn't transformed. She reverts to her true form if she dies.
Etherealness. The hag magically enters the Ethereal Plane from the Material Plane, or vice versa.
Nightmare Haunting (1/Day). While on the Ethereal Plane, the hag 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 15 (3d10). 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 the hag's soul bag. The reduction to the target's hit point maximum lasts until removed by the greater restoration spell or similar magic.