Brigmander, The Wyrm of War

Tides of War (Recharges after a Short or Long Rest). If Brigmander would be reduced to 0 hit points, his current hit points resets to 450 hit points, recharges his Annihilation Breath and he regains any expended uses of Legendary Resistance. Additionally, Brigmander can now use the options in the "Mythic Actions" section for 1 hour. Award the party an additional 155,000 XP (310,000 XP total) for defeating Brigmander after his Tides of War activates. 

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

Multiattack. Brigmander can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) force damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of Brigmander's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 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 Brigmander's Frightful Presence for the next 24 hours.

Annihilation Breath (Recharge 6). Brigmander exhales the destruction of war in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) force damage on a failed save, or half as much damage on a successful one. An object or building hit by this breath weapon takes double damage. 

Barrage Breath (Recharge 4-6). Brigmander expels arrows, crossbow bolts and other projectiles in a 100-foot line that is 10 feet wide. Each creature in that area must make a DC 24 Dexterity saving throw, taking 56 (16d6) piercing damage on a failed save, or half as much damage on a successful one. If a creature fails the saving throw by 10 or more, the projectiles pin them to the ground, reducing their speed to 0 until the end of their next turn. 

Brigmander 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. Brigmander regains spent legendary actions at the start of his turn.

Detect. Brigmander makes a Wisdom (Perception) check.

Tail Attack. Brigmander makes a tail attack.

Brandish the Fallen (Costs 2 Actions). Brigmander lets loose a torrent of bones in a line 100 feet lone and 10 feet wide. Any creature caught in this path must make a DC 24 Dexterity saving throw, taking 18 (3d12) bludgeoning damage on a failed save or half as much on a successful one. Additionally, 1d6 skeletons rise from the bones in unoccupied locations within 5 feet on the line. These skeletons are under Brigmander's control and take their actions right after him. 

If Brigmander's Tides of War trait has activated within the last hour, he can use the following actions as legendary actions. 

Hauntings of Combat (Costs 3 Actions). Brigmander can target one creature within 60 feet of him. That creature must make a DC 21 Wisdom saving throw or become cursed. The next time a cursed creature makes an attack against another creature, they automatically miss and become frightened until the start of their next turn.  

 Spike of Demise (Costs 2 Actions). Brigmander causes a steel spike to spring up form the ground underneath one creature within 120 feet of him. The targeted creature must make a DC 21 Dexterity saving throw or be hit be the spike. If they are hit, they take 12 (4d6) piercing damage plus 21 (5d8) necrotic damage. The spike sinks back into the ground after it hits or misses.