Ancient Corpsefreezer Dragon

Legendary Resistance (2/Day). If the dragon fails a saving throw, the dragon can choose to succeed instead.

Innate Spellcasting. The dragon's spellcasting ability is Wisdom (spell save DC 15). The deva can innately cast the following spells, requiring only verbal components:

At will: ray of frost, magic missile (1d6 + 2, deals cold dmg instead of force dmg) 

1/day: fog cloud (60 ft range)

Multiattack. The dragon may use its Frightful Presence. It then makes three attacks, one with its bite and two with its claws. It can replace any one of those with an additional claw or tail attack.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) piercing damage plus 3 (1d6) cold damage.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (1d12 + 7) slashing damage.

Tail. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 14 (2d6 + 7) bludgeoning damage.

Horn. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 12 (1d10 + 7) piercing damage.

Ice Breath (Recharge 5-6). The dragon exhales a ray of freezing liquid and hail in a 90-foot long, 5-foot wide line. Each creature in that area must make a DC 20 Dexterity saving throw, taking 49 (11d8) cold damage and have their speed decreased by 10 ft for 1 round on a failed save, or half as much damage and not have their speed decreased on a successful one.

The dragon can take 2 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.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Freezing Wing Attack (Costs 2 Actions). The dragon breaths a bit of ice on its wings before beating them with enough force to send shards of ice flying. Each creature within 10 feet of the dragon must succeed on a DC 20 Dexterity saving throw or take 14 (2d6 + 6) bludgeoning damage. Each creature within 25 ft of the dragon must succeed on a DC 19 Dexterity saving throw, taking 5 (1d10) cold damage and have their speed decreased by 5 ft for 1 round on a failure, or half as much damage and not get slowed on a success. The dragon can then fly up to half its flying speed.