Alatreon, The Blazing Black Dragon

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

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

Legend's Limit. Alatreon cannot perform the same Legendary Action twice in a single round.

Elemental Channeling. When Alatreon preforms a physical attack, Alatreon has the option to make the attack deal elemental damage (depending on Alatreon's current active element) rather than bludgeoning/piercing/slashing damage.

Elemental Activity. After every 2 minutes in combat, Alatreon has the choice to cycle through the three elements in the order shown below as a bonus action. When Alatreon changes its active element, its elemental based attacks change damage types & Alatreon unlocks an Elementally Active Special Attack depending on which element it has switched to.

Fire Active --> Dragon Active --> Ice Active --> ∞

Fire Active Mode. Alatreon's elemental attacks do fire damage, and Alatreon can perform Hydro Cannon as a special attack.

Dragon Active Mode. Alatreon's elemental attacks do necrotic damage, and Alatreon can perform Dragon Burst as a special attack.

Ice Active Mode. Alatreon's elemental attacks do cold damage, and Alatreon can perform Explosive Icicles as a special attack

Multiattack. Alatreon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 28 (3d12 + 9) slashing damage or elemental damage depending on Alatreon's current active element.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 9) slashing damage or elemental damage depending on Alatreon's current active element.

Horns. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 41 (9d6 + 9) piercing damage or elemental damage depending on Alatreon's current active element. If the target is a creature, it must succeed on a DC 18 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of Alatreon's choice that is within 120 feet of Alatreon and aware of it must succeed on a DC 20 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 Alatreon's Frightful Presence for the next 24 hours.

Special Attacks (Recharge 4–6). Alatreon uses one of the following special attacks.

Elemental Breath. Alatreon exhales fire/ice/dragon energy in a 90-foot cone. Each creature in that area must make a DC 26 Dexterity saving throw, taking 120 (16d10) damage depending on Alatreon's current elemental active mode on a failed save, or half as much damage on a successful one.

Elemental Beam. Alatreon exhales fire/ice/dragon energy in a 120-foot line that is 10 feet wide. Each creature in that area must make a DC 26 Dexterity saving throw, taking 108 (12d12) damage depending on Alatreon's current elemental active mode on a failed save, or half as much damage on a successful one.

Lightning Bolts. Alatreon summons up to five bolts of lightning from the skies, each of which can strike a target that Alatreon can see within 120 feet of it. A target must make a DC 19 Dexterity saving throw, taking 38 (5d10) lightning damage on a failed save, or half as much damage on a successful one.

Elementally Active Special Attacks. Unique special attacks that can only be used when Alatreon is in a certain elemental active mode.

Hydro Cannon (Fire Active). Alatreon fires up to three globules of water from its mouth at any target that it can see within 120 feet of its location. Each creature in a 10-foot radius sphere centered on each point where the water globules land must make a DC 19 Dexterity saving throw. A target takes 54 (9d8) bludgeoning damage on a failed save. On impact, the water globules leave a 20 foot radius puddle that will ignite after taking fire damage. Any creature that ends their turn in the ignited puddle will take 36 (5d10) fire damage. The puddles disappear after 2 minutes or disappear immediately if Alatreon changes its elemental active mode.  

Dragon Burst (Dragon Active). Alatreon lets out a burst of dragon energy. Each creature within 30 feet of Alatreon must make a DC 26 Dexrerity saving throw. On a failure, a creature takes 81 (9d12) necrotic damage, is launched 20 feet away from Alatreon and is knocked prone. On a success, the target only takes half as much damage, isn't pushed back and isn't knocked prone.

Explosive Icicles (Ice Active). Alatreon fires up to six icicles from its mouth at any target that it can see within 120 feet of its location. A target must make a DC 19 Constitution saving throw, taking 38 (5d10) cold damage on a failed save. On impact, the icicle will remain intact and will take up the 5-foot space where it landed. if the icicle takes lightning damage, the icicle will explode. Each creature within 15 feet of the icicle must make a DC 19 Constitution saving throw, taking 39 (6d12) lightning damage on a failed save or half as much damage on a successful one. If any other explosive icicles are within the explosion's radius, they will detonate as well, but their damage will not stack with each other. The Icicles disappear after 2 minutes or disappear immediately when Alatreon changes its elemental active mode.

Alatreon can take 5 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. Alatreon regains spent legendary actions at the start of its turn. 

Move. Alatreon moves up to half of its speed.

Detect. Alatreon makes a Wisdom (Perception) check.

Attack. Alatreon makes one bite attack, one claw attack or one horns attack.

Wing Attack (Costs 2 Actions). Alatreon beats its wings. Each creature within 10 feet of Alatreon must succeed on a DC 22 Dexterity saving throw or take 18 (3d8 + 9) bludgeoning damage and be knocked prone. Alatreon can then fly up to half its flying speed.

Special Attack (Costs 3 Actions). If Alatreon's Special Attack is charged, Alatreon preforms a special attack. If Alatreon's Special Attack isn't charged, Alatreon can roll to recharge its Special Attack instead.