Gargantuan dragon, chaotic good

Armor Class 20 (natural armor)
Hit Points 297 (17d20 + 119)
Speed 40 ft., burrow 40 ft., fly 80 ft.

STR	DEX	CON	INT	WIS	CHA
27 (+8)	10 (+0)	25 (+7)	16 (+3)	15 (+2)	19 (+4)

Saving Throws Dex +6, Con +13, Wis +8, Cha +10
Skills History +9, Perception +14, Persuasion +10, Stealth +6
Damage Resistances 
Damage Immunities fire
Condition Immunities 
Senses blindsight 60 ft., darkvision 120 ft., passive Perception 24
Languages Common, Draconic
Challenge 20 (25000 XP)

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

Actions

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: +14 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing 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 18 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.

Legendary Actions

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.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.