Arch Glacor

Primordial Chill. If the Arch Glacor would take cold damage, it heals for twice the damage dealt instead, if this would bring the Arch Glacor above it's maximum hit point total, it instead gains that many temporary hit points. This effect cannot be trigged from damage caused by the Arch Glacor or its allies.

Immutable Form. The Arch Glacor is immune to any spell or effect that would alter its form.

Death Burst. When the Arch Glacor dies, it explodes in a burst of ice. Each creature within 30 feet of it must make a DC 20 Constitution saving throw, taking 14 (4d6) cold damage and 14 (4d6) piercing damage on a failed save, or half as much damage on a successful one

Legendary Resistance (3/day, 4 in lair). If the Arch Glacor fails a saving throw, it can choose to succeed instead.

Living Frost Shield. While any Glacyte is within 30 feet of the Arch Glacor, half of any damage taken by the Arch Glacor is instead dealt to the nearest Glacyte, continuing until there is no more damage to allocate, or there are no more Glacytes to allocate to, at which point any remaining damage is dealt to the Glacor as normal.

Multiattack. The Arch Glacor makes three attacks with Ice Strike or Slam in any combination, then it can use its Exposed Core.

Ice Strike. Ranged Spell Attack: +12 to hit, range 120 ft. one target. Hit: 13 (2d6 + 6) piercing damage, and 7 (2d6) cold damage.

Slam. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 27 (3d10 + 10) bludgeoning damage.

Exposed Core (Recharge 4-6). The Arch Glacor launches its core to a point within 60 feet where it creates a wall of ice that takes the form of a 30-foot radius sphere and a thickness of 1 foot. The wall has an AC of 12 and 30 Hit Points per 10-foot section, and it has Immunity to Cold, Poison, and Psychic damage and Vulnerability to Fire damage. Reducing a 10-foot section of wall to 0 Hit Points destroys it.
Each other creature that starts its turn within the sphere must make a DC 16 Constitution saving throw or take 24 (10d4) cold damage, or half the damage on a successful save.
Additionally, each other creature within the sphere must expend twice as much movement when moving. The core remains exposed until the Arch Glacor takes 100 damage, or withdraws it as a bonus action.

Frost Cannon (Recharge 6). The Arch Glacor releases a fragment of its frost core  which emerges from its body to a point within 15 feet of itself and hovers there. This fragment becomes the Frost Cannon and immediately starts to charge up an attack.

The Frost Cannon is an object with an AC of 16, 100 hit points, and is Immune to cold, psychic and poison damage. If it is not destroyed by the start of the Arch Glacor's next turn, it unleashes a powerful beam of cold energy, destroying itself in the process.
If the cannon is subjected to Dispel Magic, the spell save DC is 20. On a failed save the cannon takes 50 damage. While the cannon is Bloodied, creatures hit by the beam have resistance to the damage

The beam of cold forms a 120-foot line that is 15 feet wide which can pass through ice as if it was not there, and the Arch Glacor chooses its direction. Each creature in the line must make a DC 20 Constitution Saving Throw. On a failure, the creature takes 41 (9d8) cold damage and 41 (9d8) force damage and it can't take a reaction until the end of its next turn. Additionally, on its next turn, it must choose whether it gets a move, an action, or a bonus action; it gets only one of the three. On a successful save, the target takes half as much damage only.
The Arch Glacor is unaffected by the beam. .

If the cannon is destroyed before it fires, the Arch Glacor's Speed is reduced to zero, it is Incapacitated, and attacks against it are at Advantage, until the end of it's next turn.

This ability is not charged at the start of an encounter, and must be charged before it can be used.

Stomp. The Arch Glacor slams itself to the ground sending out a shockwave, all creatures within 10 feet of the Arch Glacor must make a DC18 STR saving throw or take 14 (4d6) thunder damage and be knocked back 10 feet. On a successful save, the creature takes half damage and is not knocked back. If the Arch Glacor is not currently on the ground, it falls up to 500 feet straight down to the ground, taking fall damage as normal.

Pillars of Ice (Recharge 5-6). The Arch Glacor raises a solid column of ice. It takes the shape of a Cylinder that is 40 feet tall with a 10-foot radius. Each creature in this area must make a DC20 Dexterity saving throw.  On a failed save the creature is knocked prone, and raised 40 feet up on top of the column. On a successful save the creature is moved to the nearest unoccupied space. Creatures of size Huge or larger automatically succeed on the saving throw.

Recharge. Trigger: Taking damage from another creature. Response: Roll a Recharge die for one ability.

Legendary Recovery. Trigger: Taking damage from another creature. Response: Roll a D6, on a result of 4-6, regain an expended Legendary Action.

Supernatural Resistance. Trigger: Taking damage from another creature. Response: The Arch Glacor gains advantage on all attacks against the triggering creature until the end of the Arch Glacor's next turn.

The Arch Glacor can take 3 legendary actions (4 in lair), 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 Arch Glacor regains spent legendary actions at the start of its turn.

Flurried Rage. The Arch Glacor makes up to 2 attacks of either Ice Strike or Slam in any combination.

Call For Aid. The Arch Glacor picks a point within 30 feet of itself, 1d4 Glacyte minions are summoned within 5 feet of the point which act on initiative 21. This Legendary action cannot be used if a Glacyte was alive since the Arch Glacor's last turn.

Frost Cannon Recharge. The Arch Glacor recharges its Frost Cannon attack. The Arch Glacor cannot take this action again for 3 rounds.