Arch Arbornorster

Legendary Resistance (4/Day). If the Arbornorster fails a saving throw, it can choose to succeed instead.

Mythic Awakening (Recharges after a Short or Long Rest). If the Arbornorster would be reduced to 0 hit points, its current hit point total instead resets to 320 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the Arbornorster can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating the Arbornorster after its Mythic Awakening activates.

Multiattack. The Arbornorster makes one Bite attack and two Claw attacks.

Bite. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 15 (1d8 + 10) slashing damage. If the target is a Large or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. The Arbornorster can have only one creature grappled in this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 16 (1d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

Breath Weapon (Recharge 5–6). The Arbornorster uses one of the following breath weapons:

Elemental Breath. The Arbornorster exhales elemental energy in a 100-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 26 (4d12) cold damage, 26 (4d12) fire damage and 33 (5d12) psychic damage on a failed save, or half as much damage on a successful one.

Knockout Breath. The Arbornorster exhales gas in a 100-foot cone. Each creature in that area must make a DC 25 Constitution saving throw. On a failed save, the creature falls unconscious for 1 minute. On a successful save, the creature has disadvantage on attack rolls and saving throws until the end of the Arbornorster’s next turn. An unconscious creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Spellcasting. The Arbornorster casts one of the following spells, using Charisma as the spellcasting ability (spell save DC 27):

At Will: Detect Magic, Detect Thoughts, Light, Mind Sliver, Minor Illusion, Prestidigitation

3/Day Each: Dissonant Whispers (level 5 version)
2/Day Each: Fly, Lightning Bolt (level 7 version)
1/Day Each: Mind Blank (cast before combat), Scrying, Synaptic Static

Change Shape. The Arbornorster magically transforms into any fey creature with a CR no higher than 17.  While retaining its mental ability scores and personality, it gains all physical abilities of the fey form including a pool of temporary hit points belonging to the form taken.  It cannot cast any spells or Legendary Actions known to the fey form, but it can use any breath weapons normally.  This transformation ends if the temporary hit points are reduced to 0 or the Arbornorster uses a bonus action to end it.  Once any assumed form is ended, the Arbornorster can not assume that particular form again until the next dawn has passed.

Fey Leap. The Arbornorster teleports up to 60 ft.  This ability can be used in all forms that the Arbornorster assumes.

While in its own form, the Arbornorster 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. The Arbornorster regains spent legendary actions at the start of its turn.

Attack. The Arbornorster makes one Claw or Tail attack.

Cantrip. The Arbornorster casts a cantrip.

Ground Stomp (Costs 2 Actions). The Arbornorster stomps its foot dramatically. Each creature within 30 feet of it must succeed on a DC 26 Strength saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone.

If the Arbornorster's Mythic Awakening trait has activated in the last hour, it can use the options below as legendary actions.

Spell Casting. The Arbornorster casts a spell with a casting time of one Magic action.

Berzerker Betrayal (Costs 2 Actions). The Arbornorster unleashes a magical burst of enchantment magic. Each creature in a 120-foot-radius sphere centered on the Arbornorster must succeed on a DC 26 Charisma saving throw or take 19 (3d12) psychic damage and immediately use their reaction to make their strongest attack against the ally closest to them, moving up to their speed to do so if melee is the strongest attack and they are in reach.  Half damage on a successful saving throw.