Anguisa, (Greatwyrm)

Amphibious. Anguisa can breathe air and water.

Flyby. Anguisa doesn't provoke Opportunity Attacks when she flies out of an enemy's reach.

Ice Walk. Anguisa can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost her extra movement.

Tunneler. Anguisa can burrow through solid rock at half her burrowing speed and leaves a 200 foot-wide, 200-foot-high tunnel in her wake.

Legendary Resistance (5/Day). If Anguisa fails a saving throw, she can choose to succeed instead.

Queen's Vengeance (Recharges after a Short or Long Rest). If Anguisa would be reduced to 0 hit points, her current hit point total instead resets to 600 hit points, she recharges her breath weapons, and she regains any expended uses of Legendary Resistance. Additionally, Anguisa can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating Anguisa after her Queen's Vengeance activates.

Limited Magic Immunity. Limited Magic Immunity. Anguisa can’t be affected or detected by spells of 5th level or lower unless she wishes to be. She has advantage on saving throws against all other spells and magical effects.

Living Shadow. While in dim light or darkness, Anguisa has resistance to damage that isn't force, psychic, and radiant. 

Shadow Stealth. While in dim light or darkness, Anguisa can take the Hide action as a bonus action.

Sunlight Weakness. While in sunlight, Anguisa has disadvantage on attack rolls, ability checks, and saving throws.

Spellcasting. Anguisa is a 20th-level spellcaster. Her spellcasting ability is Charisma (spell save DC 34, +22 to hit with spell attacks). Anguisa has the following sorcerer spells prepared:

Cantrips (at will): mage hand, fire bolt, ray of frost, acid splash, poison spray, lightning lure

1st level (4 slots): chromatic orb, shield, darkness

2nd level (3 slots): Invisibility, shadow blade

3rd level (3 slots): counterspell, dispel magic

4th level (3 slots): greater invisibility 

5th level (3 slots): enervation, hold monster, teleportation circle

6th level (2 slot): disintegrate, globe of invulnerability 

7th level (2 slot): plane shift, teleport

8th level (1 slot): dominate monster

9th level (1 slot): wish

Multiattack. Anguisa can use her Frightful Presence. She then makes three attacks: one with her bite and two with her claws.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (2d12 + 10) piercing damage plus 21 (4d12) necrotic damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Anguisa can have only one creature grappled this way at a time.

Stinger. Melee Weapon Attack: +19 to hit, reach 20 ft., one creature. Hit: 23 (2d12 + 10) piercing damage. The target must make a DC 27 Constitution saving throw, taking 52 (15d6) poison damage on a failed save, or half as much damage on a successful one.

Frightful Presence. Each creature of Anguisa's choice that is within 120 feet of the Anguisa and aware of her must succeed on a DC 27 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Anguisa's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 4-6). Anguisa uses one of the following breath weapons.

Fire Breath. Anguisa exhales fire in a 300-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 126 (36d6) fire damage on a failed save, or half as much damage on a successful one.

Acid Breath. Anguisa exhales acid in a 500-foot line that is 20 feet wide. Each creature in that area must make a DC 27 Dexterity saving throw, taking 103 (23d8) acid damage on a failed save, or half as much damage on a successful one.

Poison Breath. Anguisa exhales poisonous gas in a 300-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 112 (32d6) poison damage on a failed save, or half as much damage on a successful one.

Lightning Breath. Anguisa exhales lightning in a 500-foot line that is 20 feet wide. Each creature in that area must make a DC 27 Dexterity saving throw, taking 126 (23d10) lightning damage on a failed save, or half as much damage on a successful one.

Cold Breath. Anguisa exhales an icy blast in a 300-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 108 (24d8) cold damage on a failed save, or half as much damage on a successful one.

Shadow Breath. Anguisa exhales engulfing shadows in a 300-foot cone. Each creature in that area must make a DC 27 Constitution saving throw. On a failed save, the creature takes 97 (15d12) necrotic damage and is frightened of Anguisa for 1 minute. On a successful save, the creature takes half as much damage and isn’t frightened. A frightened creature can repeat the saving throw at end of each of its turns, ending the effect on itself on a success. A humanoid reduced to 0 hit points by this damage dies, and an undead shadow rises from its corpse and acts immediately after Anguisa in the initiative count. The shadow is under Anguisa's control.

Anguisa can take 4 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. Anguisa regains spent legendary actions at the start of her turn.

Cantrip. Anguisa casts a cantrip.

Detect. Anguisa makes a Wisdom (Perception) check.

Stinger Attack. Anguisa makes a stinger attack.

Wing Attack (Costs 2 Actions). Anguisa beats her wings. Each creature within 15 feet of Anguisa must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Anguisa can then fly up to half her flying speed.

Work in progress 

If Anguisa's Queen's Vengeance trait has activated in the last hour, she can use the options below as legendary actions.