Annwyn (Adventurer Form)

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Inspiring Presence. Friendly creatures within 30 feet of the Paladin get +7 to saving throws , and are cannot be frightened or charmed. 

Aura of Alacrity.  Friendly creatures who start their turn within 10 feet of Annwyn have their walking speed increased by 10 feet. when she is no incapacitated. 

Divine Health. Annwyn is immune to disease in this form. 

Living Legend (1/day). Annwyn, you can empowers herself with the legendsof your her deeds. As a bonus action, she gain the following benefits for 1 minute:

She is blessed with an otherworldly presence, gaining advantage on all Charisma checks.
Once on each of her turns when she makes a weapon attack and misses, she can cause that attack to hit instead.
If you fail a saving throw, she can use her reaction to reroll it. She must use this new roll.

Spellcasting. Annwyn is a 20th-level spellcaster. Her spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). Annwyn has following Paladin spells prepared:
1st level (4 slots): Guiding Bolt, Heroism,  Bless,  Command,  Compelled Duel,  Cure Wounds,  Searing Smite
2nd level (3 slots): Enhance Ability, Magic Weapon,  Aid,  Lesser Restoration,   Protection from Poison,   Zone of Truth
3rd level (3 slots): Haste, Protection from Energy,  Crusader's Mantle,  Blinding Smite,  Revivify
4th level (3 slots): Compulsion, Freedom of Movement,  Death Ward,  Staggering Smite,  Banishment
5th level (2 slots): Commune, Flame Strike,   Holy Weapon,  Banishing Smite

Smite. Annwyn uses a spell slot to deal an additional 16 (3d8) radiant damage with a 1st level slot, 22 (4d8) on a second level, or 26 (5d8) on a 3rd, 4th, or 5th level slot on a successful attack.

Multiattack. Annwyn makes four attacks with her Holy Avenger Rapier. 

Holy Avenger Rapier. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 13 (1d8 + 8) piercing damage, does an additional 11 (2d10) radiant damage against fiends and undead.

Healing Touch (5/day). The Paladin touches another creature, the create regains 28 (5d8+5) hit points, and is freed from any nonmagical disease, poison, or deafness, or spell effects.

Inspirational Smiting (2/day). If Annwyn successfully smites and opponent she may use this to inspire friendly creatures within 30 feet of her. She may distribute up to 29 (2d8+9) temporary hit points to however many friendly creatures she likes.

Change Shape. Annwyn magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into her true form (that of an ancient Silver Dragon). She is known to favor this form, her natural form, the form of an older woman, or a sentient talking silver Raven.  She reverts to her true form if it dies. Any equipment she is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the Annwyn retains her alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Her statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form that are not one of her favored forms.

Glorious Defense (5/day). If a friendly creature within 10 feet of Annwyn that she can see, is hit by an attack roll. She can give the creature a +7 bonus to its armor class. If the attack misses, she can make one weapon attack against the attacker as part of this reaction, provided the attacker is within her weapon’s range.

Annwyn 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. Annwyn regains spent legendary actions at the start of her turn.

Detect. Annwyn makes a Wisdom (Perception) check.

Holy Avenger Rapier. Annwyn makes an attack with her Holy Avenger Rapier. She may use her smite ability if the attack hits and she has remaining spell slots. 

Panache. Annwyn may make an Acrobatics check DC 15. If she succeeds she can use acrobatic skill and grace to move up to 30 feet away, she does not provoke attacks of opportunity when doing so.

If you use Annwyn's mythic trait, treat this encounter as a level 20 encounter and a level 23 encounter, awarding 75,000 experience upon completion. Read or Paraphrase the following text when Annwyn uses his mythic trait:

Emergency Transformation. When Annwyn is reduced to 0 hit points, she doesn't die normally. Instead she reverts to her natural form, that of an ancient Silver Dragon, regaining half of her hit points, and gaining temporary hit points equal to half of her starting hit points. If her draconic form cannot fit in the area, she will revert into a tiny sprite form before escaping to fight another day (PCs do not get an additional 50,000 XP if she escapes this way). She cannot use her change shape ability until she is no longer threatened.

Annwyn can use the following actions as if they were legendary actions for 1 hour after using his Mythic trait.

Healing Surge. Annwyn uses magic lingering from her human form to heal herself, regaining 28 (5d8+5) hit points.

Claw Attack. Annwyn makes a claw attack.

Breath Attack (Costs 3 Actions, Recharge 5-6). Annwyn uses one of her breath weapons.