Chromium Greatwyrm

Ferrous Awakening (Recharges after a Short or Long Rest). If the greatwyrm would be reduced to 0 hit points, their current hit point total instead resets to 400 hit points, they recharge their Breath Weapon, and they regain any expended uses of Legendary Resistance. Additionally, the greatwyrm can now cast Mass Polymorph once as a Legendary Action (Costs 3 Actions) in the next hour. Award a party an additional 90,000 XP (180,000 XP total) for defeating the greatwyrm after its Ferrous Awakening activates.

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

Draconic Immunity. The dragon cannot be put to sleep by magic.

Metalsense. The dragon can supernaturally sense non-precious metals (such as iron, steel, or bronze) within 120 feet of themself if the metal is not behind total cover. The dragon cannot be surprised by any creature using armor or weapons made from non-precious metals.

Unusual Nature. The greatwyrm doesn’t require food or drink.

Multiattack. The greatwyrm can use their Frightful Presence. They then makes three attacks: one with their Bite and two with their Claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 16 (3d10) Cold damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 19 (2d8 + 10) slashing damage. If the target is a Huge or smaller creature, they are grappled (escape DC 16) and are restrained until this grapple ends. The greatwyrm can have only one creature grappled in this way at a time.

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 23 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 Weapon (Recharge 5–6). The greatwyrm exhales freezing air in a 300-foot cone. Each creature in that area must make a DC 24 Constitution saving throw. On a failed save, the creature takes 84 (13d12) Cold damage and their speed is reduced in half. On a successful save, it takes half as much damage and their speed is not reduced. On a success or failure, the creature’s speed becomes 0 until the end of its next turn.

Paralyzing Breath. The dragon exhales a stream of icy crystal in a 300-foot cone. Each creature in that area must succeed on a DC 24 Constitution saving throw or have their Dexterity score reduced by 6 (1d12). A creature's Dexterity score is reduced until they complete a short or long rest.

Spellcasting . The greatwyrm casts one of the following spells, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 23, +15 to hit with spell attack):

1/day each: Shape Water, Pass Without Trace, Rime’s Binding Ice, Wall of Ice, Eyebite, Flesh to Stone

The greatwyrm 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 greatwyrm regains spent legendary actions at the start of their turn.

Claw. The greatwyrm makes one Claw attack.

Spellcasting (Costs 2 Actions). The greatwyrm uses Spellcasting.

Ferrous Eye (Costs 3 Actions). The greatwyrm emits a beam of draining energy in a 90-foot line that is 10 feet wide. Each creature in that area must make a DC 26 Charisma saving throw, taking 27 (5d10) Necrotic damage on a failed save, or half as much damage on a successful one.