Ancient Coldbolt Dragon

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.

Magic Weapons. The Dragon's weapon attacks are magical.

Siege Monster. The dragon deals double damage to objects and structures.

Amphibious. The dragon can breathe air and water.

Master of Cold And Lighting. If the dragon is hit by a Cold, Lightning, or Thunder damage, it instead heals it equal to the amount of damage it would have dealt.

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

Spellcasting. The Dragon casts one of the following spells, requiring no spell components and using Intelligence as the spell casting ability.
At will: Dancing Lights, Guidance
5/day each:  Rime's Binding Ice, Chaos Bolt (Lightning)
4/day: Ice Storm, 
1/day: Storm of Vengeance

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite, one with it's claws, and one with it's tail.

Bite. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 38 (3d12 + 20) piercing damage plus 18 (4d8) cold damage.

Claw. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 36 (3d10 + 20) slashing damage.

Tail. Melee Weapon Attack: +16 to hit, reach 30 ft., one target. Hit: 43 (3d12 + 20) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 16 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 the dragon's Frightful Presence for the next 24 hours.

Cold Breath (Recharge 3-4). The dragon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 84 (16d8+ 10) cold damage on a failed save, or half as much damage on a successful one.

Lightning Discharge (Recharge 5-6). The dragon's body discharges with lightning in a 150 ft sphere. Each creature in that area must make a DC 20 Constitution saving throw, taking 109 (14d12+ 20) Lighting damage on a failed save, or half as much on a successful one.

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. 

Claw. The dragon makes one claw attack.
Bite. The dragon can make one bite attack.
Lightning Discharge. The dragon uses it's Lightning discharge if it's able.