Valigarmanda, Skyruin Serpent

Air Superiority. Valigarmanda does not provoke opportunity attacks when it flies out of an enemy's reach. Valigarmanda also cannot fall in elevation unless it wishes too, even if it is incapacitated.

Legendary Resistances (5/Day). If Valigarmanda fails a saving throw, it can choose to succeed instead.

Phase Shift. Valigarmanda has different phases it switches between as it loses hit points. The phases decide what type of multi attack and legendary actions it takes. It starts in Ice phase and after losing 200 hit points it switches to lightning phase. When it loses 200 hit points in lightning phase it switches to fire phase. Whenever Valigarmanda falls beneath the health point threshold, it immediately takes a special reaction which switches its phase and uses Skyruin. It can make this reaction even if it's taken a reaction in that round already.

Raid Boss. Valigarmanda is immune to any effects that would change its shape. When it attacks an object, it always deals the maximum damage possible and doubles all damage it deals to objects. Damage Valigarmanda deals to other creatures also ignores resiatances and treats immunities as if they were resistances.

Turbulence. The air within 200 feet of Valigarmanda is turbulent and becomes difficult terrain for creatures that attempt to fly through it. Creatures that take fall damage within 200 feet of Valigarmanda take double the fall damage.

Multiattack. Valigarmanda can use its Triscourge. It then makes three attacks: two with its claws and one with its bite.

Bite. Weapon Attack: + 19 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 10) piercing damage plus 12(4d6) damage that depends on its phase. Cold damage for Ice phase, lightning damage for Lightning phase, and fire damage for Fire phase.

Claw. Weapon Attack: + 19 to hit, reach 15 ft., one target. Hit: 24 (4d6 + 10) slashing damage.

Triscourge. Valigarmanda causes a calamity based on which phase it is in. It uses Calamity's Chill in Ice phase, Calamity's Bolt in Lightning phase, and Calamity's Inferno in Fire phase. 

Calamity's Chill (Ice Phase). Valigarmanda attempts to encase a creature within 100 feet of it in a pillar of ice. That creature must succeed on a DC 27 Constitution saving throw taking 40(8d10) cold damage and becoming incapacitated, as they are trapped in a pillar of ice on a failure. On a success, they take half as much damage and aren't trapped in ice. The ice pillar has 20 hit points, an AC of 20 and has vulnerability to fire damage, but it has immunity to psychic damage. The ice takes all damage for the creature trapped inside and takes up their space, but when it is reduced to 0 hit points, any left over damage is dealt to the creature that was trapped.

Calamity's Bolt (Thunder Phase). Valigarmanda causes a bolt of lightning to strike a creature of its choice within 100 feet of it. That creature must succeed a DC 27 Dexterity saving throw, taking 45(9d10) lightning damage on and becoming dazed for a minute on a failure or taking half as much damage and not being dazed on a success. A dazed creature cannot take reactions and has its movement speed halved and if it would become dazed again, it is instead stunned for the same duration. A creature can repeat this saving throw at the end of its turn to end the effect on itself. A creature that fails this saving throw by 10 or more is stunned for the duration instead.

Calamity's Inferno (Fire Phase). Valigarmanda releases a massive inferno from its beak at a point it can see within 100 feet of itself. Every other creature in a 30 foot radius sphere centered on that point must make a DC 27 Dexterity saving throw, taking 50(10d10) fire damage on a failure or half as much on a success. 

Skyruin (Only used by Phase Switch Reaction). Valigarmanda releases a massive explosion of elemental energy and switches phases. Each creature within 300 feet of Valigarmanda takes 5d10 damage of a type that depends on Valigarmanda's phase and is inflicted with a different effect for the duration of the phase.

During Ice phase, the damage dealt is cold danage and has their movement speed reduced by 10 feet for the duration of the phase.

During Lightning phase, the damage dealt is lightning and whenever a creature ends its turn within 10 feet of another creature during this phase, the creature that ended its turn becomes dazed until the end of its next turn.

During Fire phase, the damage dealt is fire and each creature is inflicted with conflagration for the duration of this phase. Conflagration deals 16(4d8) fire damage to a creature at the start of its turn and if a creature falls to 0 hit points while inflicted with conflagration, every other creature within 10 feet of that creature takes 16(4d8) fire damage. 

Spikecicle (Recharge 5-6)(Ice phase). Valigarmanda chooses any number of creatures it can see within 150 feet of them and launches an icicle at each of those creatures. Valigarmanda makes a spell attack (+19 to hit) against each of those creatures, dealing 35(5d10+10) cold damage on hit. Even if the icicles miss, each creature that was attacked must make a DC 27 Dexterity saving throw, taking 50(10d10) cold damage on a failure and having their movement speed reduced to 0 during their next turn. On a success, they take half damage and don't have their movement speed reduced. 

Thunderous Breath (Recharge 5-6)(Lightning Phase). Valigarmanda exhales lightning in a 150-foot cone. Each creature in that line must make a DC 27 Dexterity saving throw, taking 100 (20d10) lightning damage and becoming dazed for a minute on a failed save. On a success they take half damage and aren't dazed on a successful one. A creature can repeat this saving throw at the end of its turns to end the daze on them.

Mountain Fire (1/Day)(Fire Phase). Valigarmanda summons a pillar of flames in a space it can see within 300 feet of itself. The pillar is takes up a 10 foot cube and each creature in that space when it appears or ends its turn in the cube, takes 25(5d10) fire damage. On following turns, Valigarmanda can use one of its attacks to attack the pillar and cause one of two different effects to happen. If there is a creature in a space adjacent to the pillar of flames, that creature takes 200 damage divided by their Constitution modifier. If there is no creature adjacent to the pillar, or there are two or more creatures adjacent to the pillar, each creature within 100 feet of the pillar must succeed a DC 27 Dexterity saving throw taking 100(20d10) fire damage on a failure or half as much on a success.

Slashing Gale. As a reaction to being attacked, Valigarmanda can grant that attack disadvantage, and deal 20(4d10) thunder damage to the attacking creature of they are within 60 feet of Valigarmanda.

Phase Switch. As a reaction, to losing 200 hit points or rolling initiative, Valigarmanda uses Skyruin. The first time it uses it, it shifts into Ice phase.

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

Bite. Valigarmanda makes a bite attack.

Downdraft. Valigarmanda causes a powerful down draft which forces every creature flying within 150 feet of it to make a DC 27 Constitution saving throw or be knocked prone and pushed towards the ground on a failure.

Maneuver. Valigarmandra moves up to half its movement speed. 

Triscourge(2 Actions). Valigarmandra uses the Calamity of the phase it is in.