Actions

Multiattack. The dragon can use its Frightful Presence. It then makes five attacks: two with its bite and three with its claws.   Bite. Melee Weapon Attack: +19 to hit, reach 15 ft, one target. Hit: 23 4d12+10 piercing + 9 4d10 acid damage.
Claw. Melee Weapon Attack: +19 to hit, reach 10 ft, one target. Hit: 17 4d8+10 slashing damage.
Tail. Melee Weapon Attack: +19 to hit, reach 20 ft, one target. Hit: 19 3d10+10 bludgeoning damage.   Frightful Presence. Each creature of the dragon’s choice that is within 160 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 6 rounds.   Necrotic Breath (Recharge 3-4). The dragon exhales necrotic mist in a 120-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw, taking 113 25d8 necrotic damage on a failed save, or half as much damage on a successful one.   Acid Breath (Recharge 3-4). The dragon exhales acid in a 120-foot line that is 15 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 130 20d12 acid damage on a failed save, or half as much damage on a successful one.   Oblivion Breath (Recharge 3, 2nd Phase). An acrid beam erupts from the dragon's mouth in a 5-foot-wide, 120-foot-line. Each creature within line must make a DC 23 Dexterity saving throw, taking 78 12d12 acid + 78 1d12 untyped damage on a failed save and half as much on a successful one.

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 23 Dexterity saving throw or take 23 4d8+10 bludgeoning damage and be knocked Prone. The dragon can then fly up to half its flying speed.