Arkalchos, Dragon of Frozen Time

Ice Walk. The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

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

Foe of the Sun. When the Arkalchos takes radiant damage from an effect that uses dayligh, or contains the word "Sun" in its name, the damage is doubled on top of the voulnerability.

Pain of the Living Light. When Arkalchos takes radiant damage, the entire plane quakes with his agony. every creature in the realm, has to succeed on a DC 15 DEX save, or be knocked prone, any structure, like a fortress or similar, loses half it's hitpoints, and crumbles completely on a repeated occurence.

Armor of Absolute Zero. Any time this creature is hit by a mele attack, the creature that hit it must succeed on a DC22 Saving throw, or become stunned, untill the end of it's next turn.

Savage Attacker. The dragon rolls critical hits on rolls of 19 and 20 on the
d20. Once per turn, it may reroll a damage die and take either
total.

Flyby. The dragon doesn't provoke opportunity attacks when it
flies out of an enemy's reach.

Terrifying. When the dragon uses its Dreadful ancient aura, it immediately makes an attack against a creature within reach.

Multiattack. The dragon 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 25 ft., one target. Hit: 32 (4d10 + 10) piercing damage plus 22 (4d10) cold damage.

Claw. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 30 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage.

Dreadful Ancient Aura. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 18 Wisdom saving throw or gain a level of Dread. An affected creature can repeat this saving throw at the end of it's turn to reduce its Dread level by 1.

Cold  Flame Breath (Recharge 5–6). The dragon exhales blast of blue flame in a 20 foot wide and 180 foot long line. Each creature in that area must make a DC 26 Constitution saving throw, taking 50 (11d8) cold damage and 50 (11d8) fire damage on a failed save, or half as much damage on a successful one.

Heat Death (Recharge 5–6). Every creature in a 60 foot radius of the dragon makes a DC 26 Constitution Saving throw, becoming frozen on a failure. Frozen creatures suffer from the petrified condition

Expulsion of Heat. After the dragon has used the heat death ability for a 3rd time, it can cause a release of this stored up energy. every creature in a 40 fot radius must succeed on a DC 25 Dexterity Saving throw, or take 49 (14d6) points of fire damage.

The dragon 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 dragon regains spent legendary actions at the start of its turn.

Manifest Self. The dragon ends one condition affecting it

Tail Attack. The dragon makes a tail attack.

Transposition (Costs 2 Actions). The dragon's wings blur, and it dissapears, it reapears in the same instant at any point within it's realm.

Heat death (Costs 3 Actions). Arkalchos uses the Heat Death action.