gargantuan Dragon
Armor Class 20 (natural armor)
Hit Points 301
Speed fly +80, swim +40, walk +40, burrow +40
Challenge 17
Saving Throws cha +9, con +14, dex +6, int +0, str +8, wis +9
Damage Immunities cold
Senses darkvision 120 ft., keensense 60 ft.
Languages Common,Draconic
Ice Walk. The dragon can move across and climb icy surfaces without needing to make an ability check. In addition, difficult terrain composed of ice or snow doesn't cost it extra movement.
Legendary Resistance (3/Day). If the dragon fails a save, it can choose to succeed instead.
Numbing Chill. When the white dragon hasn't unleashed its Cold Breath, the cold builds up within it and ripples outward from it. While the dragon's Cold Breath is available, it emits a numbing chill, and each creature that starts its turn within 30 feet of the dragon must succeed on a DC 22 CON save or its speed is halved.
Multiattack. The dragon uses its Frightful Presence. It then makes one Bite attack and two Claw attacks.
Bite. _Melee Weapon Attack:_ +14 to hit, reach 15 ft., one target. _Hit:_ 19 (2d10+8) piercing damage plus 9 (2d8) cold damage.
Claw. _Melee Weapon Attack:_ +14 to hit, reach 10 ft., one target. _Hit:_ 15 (2d6+8) slashing damage.
Tail. _Melee Weapon Attack:_ +14 to hit, reach 20 ft., one target. _Hit:_ 17 (2d8+8) 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 17 WIS save or be frightened for 1 minute. A creature can repeat the save at the end of each of its turns, ending the effect on itself on a success. If a creature's save 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 5-6). The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 22 CON save, taking 63 (14d8) cold damage on a failed save, or half as much damage on a successful one.
The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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 automatically detects every creature and trap within 30 feet of it, pinpointing the current location of each.
Tail Attack. The dragon makes a Tail attack.
Elemental Roar (Costs 2 Actions). The dragon roars at up to two creatures of its choice within 120 feet of it. Each target must succeed on a DC 17 CHA save or be vulnerable to cold damage until the end of its next turn. A target resistant to cold damage isn't resistant to it for the duration. A target immune to cold damage is still immune to it, even if the target fails the save.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 22 DEX save or take 15 (2d6+8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed without provoking opportunity attacks.
Source: Black Flag SRD · OGL/Open