Wendigo Nightmother

Inhuman: Nightmother does not need to Breathe or sleep

Frightful Presence: 

The Nightmothers very presence puts the mind into a panic

Anyone coming within 30ft of nightmother for the first time must succeed a DC 14 Wisdom Saving throw or be frightened of the nightmother

Targets can re-attempt saving throw at the start of each of their turns, once they succeed, they cannot be frightened by the nightmothers frightful presence again

Regeneration + Degeneration: 

While Nightmother is feeding off of the captured forgotten in her den she passivly regains 10 hitpoints at the start of every turn

Alternatively, every time the party removes a captured forgotten from her range (120ft) she instead takes 6d12 damage and loses 1AC

Spellcasting. The Nightmother is a 12-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +9 to hit with spell attacks). The Wendigo Nightmother has following Warlock spells prepared:
Cantrips (at will): Eldritch Blast, Frostbite
1st level (3/day Each): Hellish Rebuke, Counterspell

Multiattack: Claw Slash: Melee Weapon Attack: +5 to hit (Str), 1 target. Hit: 16 (2d10 + 7) Slashing damage.

Additionally, when this attack hits target, it deals (1d6) Poison Damage for (1d4) Rounds, unless target can succeed a DC15 Constitution Saving throw

Target can re-attempt Saving throw at the start of each turn

Pounce: Ranged Weapon Attack: +7 to hit (Dex), range 15ft ft., 1 target. Hit: 17 (2d12 + 7)  Slashing damage.

Target must also succeed a DC17 Strength Saving Throw on success target is pushed back 15ft, on failure target is pushed back 15ft and knocked prone

Should target collide with any other target or object both will take 1d4 additional damage

If target fails saving throw, Nightmother is able to make an additional pounce attack towards a different target within range

Bite: Melee Weapon Attack: +5 to hit (Str), 1 Target, Hit:  (3d8 + 5) Peircing Damage + (2d6) Necrotic Damage

Cling: Reaction (dodge): 

Nightmother can choose to dodge an attack by leaping onto the ceiling out of harms way

After Nightmother uses this ability it must be recharged

Roll a d6 on the start of every turn on a 5 or 6 ability is recharged and can be used again

Root Eruption (Costs 5 Actions): 

Nightmother lets out a horrific shreak, causing the roots in her den to go haywire (5 Legendary Action Charges)

Once this ability has been used it must be recharged, roll a d20 on an 19 or 20 ability is recharged and can be used again

Roots extrude from the ceiling, and walls, all spaces within 10ft, centered on pillar become difficult terrian, pillars are place randomly around den with a maximum pillar count being 15, party can expend an action and attempt a DC12 Nature Check to remove the pillar, party cannot remove roots surrounding pillar

All targets immediately take 3d10+8 unless they can succeed a DC22 Dexterity Saving Throw

Root Passage (Costs 2 actions): 

Nightmother calls upon the ancient and twisted roots to protect her from harm and give her the element of surprise

She can use this legendary action to immediately get dragged into the roots below her almost instantly

Nightmother can re-appear anywhere within 30ft of passage, should she choose to appear from the floor within an occupied space, she can make 1 surprise attack (slash) against target

If she choose to appear from the ceiling, or walls she can make 1 pounce attack

Does not provoke opprotunity attack

Legendary Resistance (3/day): Should Nightmother fail a saving throw, she can choose to pass instead