Apocalypse Doomwyrm

Legendary Resistance (7/Day). If the Apocalypse Doomwyrm fails a saving throw, it can choose to succeed instead.

Apocalypse awakening. If the Apocalypse Doomwyrm is reduced to 0 hit points, it instead goes to 1000 hit points, its breath weapon recharges, and it is able to cast the spell Meteor Swarm instead of Frightful presence. Additionally, the Apocalypse Doomwyrm can now use the options in the "Mythic Actions" section for 1 hour.

Being of destruction. If the Apocalypse Doomwyrm brings a creature to 0 hit points, the creature at 0 hit points starts with 1 failed death save. The Apocalypse Doomwyrm is also immune to spells and abilities that instantly kill creatures(example: power word kill). 

Unkillable. If the Apocalypse Doomwyrm is slain, its soul refuses to accept its fate and lives on as a disembodied spirit that fashions a new body for itself after 100d100 years. The Apocalypse Doomwyrm's soul can fashion a new body even if its old body was burned to ash or otherwise obliterated. When the new body is complete, the Apocalypse Doomwyrm regains all his hit points and becomes active again. The Apocalypse Doomwyrm's new body appears In a random plane, but not the one it was slain in.

Unchanging. The Apocalypse Doomwyrm cannot have its form changed in any way.

Magic Body. All of the Apocalypse Doomwyrm's attacks count as magical.

Regeneration. At the start of each of the Apocalypse Doomwyrm's turns, it heals 15 hp. 

Multiattack. The Apocalypse Doomwyrm can use its Frightful Presence. It then makes five attacks: one with its bite, one with its tail, and three with its claws.

Bite. Melee Weapon Attack: +25 to hit, reach 25 ft., one target. Hit: 86 (12d10 + 20) piercing damage.

Claw. Melee Weapon Attack: +25 to hit, reach 15 ft., one target. Hit: 60 (10d6 + 25) slashing damage.

Tail. Melee Weapon Attack: +25 to hit, reach 30 ft., one target. Hit: 82 (15d8 + 15) bludgeoning damage.

Frightful Presence. Each creature of the Apocalypse Doomwyrm's choice that is within 150 feet of the Apocalypse Doomwyrm and aware of it must succeed on a DC 31 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 Weapons (Recharge 5–6). The Apocalypse Doomwyrm uses one of the following breath weapons.

Force Breath. The Apocalypse Doomwyrm exhales a beam force in a 300-foot line that is 30 feet wide. Each creature in that area must make a DC 31 Dexterity saving throw, taking 212 (35d10+20) force damage on a failed save, or half as much damage on a successful.

Weakening Breath. The Apocalypse Doomwyrm exhales gas in a 300-foot cone. Each creature in that area must succeed on a DC 31 Constitution saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Pulsar (Recharge 20). The Apocalypse Doomwyrm Produces a massive explosion centered on itself. Each creature within 75 feet of the Apocalypse Doomwyrm must make a DC 31 Dexterity saving throw, taking 310 (20d20+100) force damage on a failed save, or half as much damage on a successful one. All creatures who take damage from this ability are blinded and deafened for 1d4 turns.

The Apocalypse Doomwyrm can take 5 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 Apocalypse Doomwyrm regains spent legendary actions at the start of its turn.

Detect. The Apocalypse Doomwyrm makes a Wisdom (Perception) check.

Claw Flurry (Costs 2 Actions). The Apocalypse Doomwyrm makes 2 Claw attacks.

Wing Attack (Costs 2 Actions). The Apocalypse Doomwyrm beats its wings. Each creature within 15 feet of the Apocalypse Doomwyrm must succeed on a DC 31 Dexterity saving throw or take 65 (13d6+20) bludgeoning damage and be knocked prone. The Apocalypse Doomwyrm can then fly up to half its flying speed.

Multiversal Tear (Costs 5 Actions). The Apocalypse Doomwyrm rips an unstable hole through spacetime. A black hole with a 20ft diameter appears  in a space within 100ft of the Apocalypse Doomwyrm. All creatures except for the Apocalypse Doomwyrm within 200ft of the black hole must make a DC 31 Strength saving throw or get pulled a number of feet equal to 5 times the number the creature failed the check by, towards the center of the black hole. Creatures also have to make the save at the end of their turn. An immobile object automatically fails the save by 20. If a creature is pulled upwards by the black hole, it has disadvantage on Dexterity and Strength saving throws while in the air, unless they have a flying speed. A creature that starts their turn inside the black hole takes 262 (25d20) force damage and is blinded. The black hole disappears at the end of the creature after the Apocalypse Doomwyrm's turn, unless the Apocalypse Doomwyrm uses its legendary actions to maintain it.