Belthora, The Wretched Hag Mother

Amphibious. Belthora can breathe air and water.

Eldritch Mind. Belthora has advantage on Constitution saving throws made to keep concentration on a spell or other effect. 

Fey Resurgence. If Belthora would be reduced to 0 hit points, her current hit point total instead resets to 300 hit points, she recharges her "", and she regains any expended uses of Legendary Resistance. Additionally, Belthora can now use the options in the "Mythic Actions" section for 1 hour, gaining 2 more Legendary Actions per turn (5 Legendary Actions total) and any expended actions are regained. Award a party an additional 75,000 XP (150,000 XP total) for defeating Belthora the Wretched Hag Mother after her Fey Resurgence activates.

Legendary Resistance (4/Day). If Belthora fails a saving throw, she can choose to succeed instead.

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

Mimicry. Belthora can mimic animal sounds and humanoid voices. A creature that hears the sounds can tell they are imitations with a successful DC 33 Wisdom (Insight) check.

Multiattack. Belthora makes three attacks: two with her Witchfire and one with her Eldritch Claw. Alternatively, she makes one attack with her Eldritch Claw or Witchfire and casts one spell.

Eldritch Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 6) slashing damage and 28 (5d10) force damage. If the target is Medium or smaller, it is grappled (DC 16 to escape). A target grappled this way is also restrained.

Witchfire. Ranged Weapon Attack: +15 to hit, reach 120 ft., one target. Hit: 29 (4d10 + 7) fire damage.

Change Shape. Belthora 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 is transformed. She reverts to her true form if she dies.

Spellcasting. Belthora's spellcasting ability is Charisma (spell save DC 23; +15 to hit with spell attacks). She can cast the following spells, requiring no material components:

At will: dancing lights, minor illusion, vicious mockery

Invisible Passage. Belthora magically turns invisible until the end of her next turn. While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic. Any equipment she wears, or carries is invisible with her.

Counterspell (Uses a 3rd level or higher spell slot). Belthora attempts to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, she must make an ability check using her Charisma modifier (+7). The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect. On a failure, the spell still takes effect, but Belthora does not lose her reaction as normal.

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

Attack. Belthora makes an attack with her Eldritch Claw or Witchfire.

Cast a Spell (Costs 2 Actions). Belthora casts a spell of 5th level or lower.

Disrupt Life (Costs 3 Actions). Each non-undead creature within 20 feet of Belthora must make a DC 20 Constitution saving throw against this magic, taking 21 6d6) necrotic damage on a failed save, or half as much damage on a successful one.

If Belthora’s Fey Resurgence trait has activated in the last hour, she can use the options below as legendary actions.

Maddening Touch (Costs 2 Actions). Belthora targets a creature she is grappling. The creature must succeed on a DC 25 Charisma saving throw or take 44 (8d10) psychic damage and be banished to one of the nonsensical forests of the Feywild. At the start of Belthora's next turn, the creature reappears in an unoccupied space within 10 feet of her.

Queen Mab's Scream (Costs 3 Actions). Belthora lets out a guttural scream that tears through anyone who can hear it. Each creature in a 300-foot-radius sphere centered on her must make a DC 25 Constitution saving throw. On a failed save, the creature takes 39 (6d12) thunder damage and is deafened for 1 minute. Then each creature in the same radius must make a DC 25 Wisdom saving throw (DC 18 if they succeeded on the Constitution saving throw). On a failed save, the creature is stunned for 1 minute. A creature can repeat these saving throws at the end of their turns, ending the effect on itself on a success.