Ancient Silver Dragon

4/day (5/day in lair). If the ancient silver dragon fails a saving throw, it can choose to succeed instead.

The ancient silver dragon makes three Rend attacks. It can replace one attack with a use of (A) Paralyzing Breath or (B) Spellcasting to cast (level 2 version).

Recharge 5–6. Constitution Saving Throw: DC 24, each creature in a 90-foot Cone.
Failure: 67 (15d8) cold damage.
Success: Half damage.

Melee Weapon Attack: +17 to hit, reach 15 ft.. Hit: 19 (2d8 + 10) slashing plus 9 (2d8) cold damage.

Constitution Saving Throw: DC 24, each creature in a 90-foot Cone.
First Failure: The target has the Incapacitated condition until the end of its next turn, when it repeats the save.
Second Failure: The target has the Paralyzed condition, and it repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.

1 use per day. The ancient silver dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 23, +15 to hit with spell attacks):
At Will: Detect Magic, Hold Monster, Ice Knife (level 2 version), Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell)
1/Day Each: Control Weather, Ice Storm (level 7 version), Teleport, Zone of Truth

The ancient silver dragon uses Spellcasting to cast Hold Monster. The dragon can't take this action again until the start of its next turn.

Dexterity Saving Throw: DC 23, each creature in a 60-foot-long, 10-foot-wide Line.
Failure: 14 (4d6) cold damage, and the target is pushed up to 30 feet straight away from the ancient silver dragon.
Success: Half damage only.
Failure or Success: The ancient silver dragon can't take this action again until the start of its next turn.

The ancient silver dragon moves up to half its Speed, and it makes one Rend attack.