Aetherion, The Convergence Dragon

Convergence Aura: Aetherion is a living nexus of all elemental forces. Within a 300-foot radius, the environment continuously shifts, mimicking the effects of various elemental planes. Each round, creatures in this area must succeed on a DC 25 Constitution saving throw or suffer 2d10 damage of a randomly determined type (acid, cold, fire, lightning, poison, psychic, radiant, or thunder).

Adaptive Resistance: Aetherion is resistant to all damage types. Upon taking damage from a specific type (acid, cold, fire, lightning, poison, psychic, radiant, thunder), it becomes immune to that damage type until the end of its next turn.

Elder Will: Aetherion's mind is ancient and unfathomable. It has advantage on saving throws against being charmed, frightened, or stunned. Additionally, any creature that attempts to read its thoughts or mentally communicate with it must succeed on a DC 26 Wisdom saving throw or be stunned for 1 minute.

Multiattack: Aetherion can use its Frightful Presence. It then makes three attacks: one with its Bite and two with its Claws.

Bite: Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) piercing damage plus 16 (3d10) elemental damage (choose one: acid, cold, fire, lightning, poison, psychic, radiant, or thunder).

Claw: Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Tail: Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Breath of Convergence (Recharge 5-6): Aetherion exhales a torrent of raw elemental energy in a 150-foot cone. Each creature in that area must make a DC 28 Dexterity saving throw, taking 110 (20d10) damage of a randomly determined type (acid, cold, fire, lightning, poison, psychic, radiant, or thunder) on a failed save, or half as much damage on a successful one.

Frightful Presence: Each creature of Aetherion's choice within 240 feet of it and aware of it must succeed on a DC 26 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if Aetherion is within line of sight, ending the effect on itself on a success.

Planar Shift: Aetherion can teleport up to 120 feet to an unoccupied space it can see. This movement does not provoke opportunity attacks. After using this ability, Aetherion’s next attack or saving throw has advantage.

Elemental Counter: When Aetherion takes damage from a spell, it can use its reaction to unleash a burst of elemental energy. Each creature within 30 feet of Aetherion must succeed on a DC 26 Dexterity saving throw or take 22 (4d10) damage of a type (acid, cold, fire, lightning, poison, psychic, radiant, or thunder) chosen by Aetherion.

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

• Detect: Aetherion makes a Wisdom (Perception) check.
• Tail Attack: Aetherion makes a tail attack.
• Wing Attack (Costs 2 Actions): Aetherion beats its wings. Each creature within 20 feet of Aetherion must succeed on a DC 27 Dexterity saving throw or take 20 (4d6 + 10) bludgeoning damage and be knocked prone. Aetherion can then fly up to half its flying speed.

When Aetherion's hit points are reduced to 0 for the first time, it does not die or fall unconscious. Instead, it becomes infused with pure elemental power, regaining 500 hit points and gaining access to the following mythic actions:

• Elemental Overload (Costs 3 Actions): Aetherion releases a devastating surge of energy. Each creature within 300 feet of it must make a DC 28 Constitution saving throw, taking 100 (20d8) damage of a type chosen by Aetherion on a failed save, or half as much on a successful one.
• Reality Tear (Costs 3 Actions): Aetherion tears a rift in reality itself. Choose a 60-foot area within 240 feet. All creatures in that area must succeed on a DC 28 Dexterity saving throw or be pulled into the rift, taking 75 (15d10) force damage and being teleported to a random plane of existence. The rift then closes.