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

Undead Reawakening (Recharges after a Short or Long Rest)   If Ytontyss would be reduced to 0 hit points, its current hit point total instead resets to 450 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, Ytontyss can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating the greatwyrm after its Metallic Awakening activates.

Magic Resistance Ytontyss has advantage on Saving Throws against spells and other Magical Effects.

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: +18 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Ytontyss can only have one creature grappled in this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

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 25 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.

Breath Weapons (Recharge 5–6). Ytontyss uses one of the following breath weapons.

Lifeless Conflagration. Ytontyss exhales undead flame in a 300-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 84 (13d12) fire and necrotic damage on a failed save, or half as much damage on a successful one.

Weight of Ages. Ytontyss exhales gas in a 300-foot cone. Each creature in that area must make a DC 26 Strength saving throw. On a failed save, the creature falls unconscious for 1 minute. On a successful save the creature has disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Change Shape. Ytontyss magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses its action to end it.

Ytontyss 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. Ytontyss regains spent legendary actions at the start of its turn.

Detect. Ytontyss makes a Wisdom (Perception) check.

Attack. Ytontyss makes one Claw or Tail attack.

Wing Attack (Costs 2 Actions). Ytontyss beats its tattered wings. Each creature within 15 feet of the dragon must succeed on a DC 26 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Ytontyss can then fly up to half its flying speed.

If Ytontyss' Undead Reawakening trait has been activated in the last hour, it can use the options below as legendary actions.

Bite. Ytontyss makes one Bite Attack.

Wail of the Damned (Costs 2 Actions). Ytontyss wails with the agonies of the undead. Each creature within a 120-foot-radius sphere centered on Ytontyss must succeed on a DC 26 Constitution saving throw or take 19 (3d12) necrotic damage and be incapacitated until the end of its next turn.