Apocalypse

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

Magic Resistance. Apocalypse has advantage on saving throws against spells and other magical effects.

Reflective Carapace. Any time Apocalypse is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, Apocalypse is unaffected. On a 6, Apocalypse is unaffected, and the effect is reflected back at the caster as though it originated from Apocalypse, turning the caster into the target.

Siege Monster. Apocalypse deals double damage to objects and structures.

Amphibious. It can breathe air and water. can breathe air and water.

Ice Walk. Apocalypse can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

Regeneration. Apocalypse regains 200 hit points at the start of its turn. It dies only if it starts its turn with 0 hit points and doesn't regenerate or it takes more than double its hp as damage. Both its regrowing of heads and healing abilities can only be halted by a wish spell. 

Multiple Heads. Apocalypse has ten heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever Apocalypse takes 100 or more damage in a single turn, one of its heads dies. If all its heads die, Apocalypse dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn. Apocalypse regains 40 hit points for each head regrown in this way.

Reactive Heads. For each head Apocalypse has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Antimagic Cone. Apocalypse has a central eye on its chest that creates an area of antimagic, as in the antimagic field spell, in a 300-foot cone. At the start of each of its turns, Apocalypse decides which way the cone faces and whether the cone is active. The area works against the its own eye rays.

Natural Disaster. If Apocalypse is flying, all creatures in a 600 feet radius musst make a DC 22 Strength Save or be knocked prone and 20 feet away. If it is walking, all creatures in a 600 feet radius musst make a DC 22 Dexterity Save or be knocked prone. 

Multiattack. Apocalypse can attack with each of its heads twice per round. Once a breath attack and one ray attack. Or it can make one bite attack or swallow. All Ray attacks have a range of 300 feet. 

Ancient Black Dragon Head

Acid Breath (Recharge 5–6). The head exhales acid in a 180-foot line that is 20 feet wide. Each creature in that line must make a DC 32 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

Charm Ray. The Eyes shoot a beam at one target, which has to succeed a DC 25 Wisdom saving throw, or be charmed for 1 hour. 

Ancient Blue Dragon Head

Lightning Breath (Recharge 5–6). The head exhales lightning in a 240-­-foot line that is 20 feet wide. Each creature in that line must make a DC 33 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Paralyzing Ray. The Eyes shoot a beam at one target, which has to succeed a DC 26 Constitution saving throw, or be paralyzed for 1 minute. The saving throw can be repeated at the end of the targets turn, ending the effect on a success.

Ancient Brass Dragon Head

Fire Breath(Recharge 5–6). The head exhales fire in an 180-foot line that is 20 feet wide. Each creature in that line must make a DC 31 Dexterity saving throw, taking 56 (16d6) fire damage on a failed save, or half as much damage on a successful one.

Sleep Breath(Recharge 5–6). The head exhales sleep gas in a 180-foot cone. Each creature in that area must succeed on a DC 31 Constitution saving throw or fall unconscious for 10 minutes. This effect ends for a creature if the creature takes damage or someone uses an action to wake it.

Fear Ray. The targeted creature must succeed on a DC 26 Wisdom saving throw or be frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Ancient Bronze Dragon Head

Lightning Breath(Recharge 5–6). The head exhales lightning in a 240-foot line that is 20 feet wide. Each creature in that line must make a DC 33 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Repulsion Breath(Recharge 5–6). The head exhales repulsion energy in a 60-foot cone. Each creature in that area must succeed on a DC 33 Strength saving throw. On a failed save, the creature is pushed 60 feet away from the creature.

Slowing Ray. The targeted creature must succeed on a DC 26 Dexterity saving throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Ancient Copper Dragon Head

Acid Breath(Recharge 5–6). The head exhales acid in an 180-foot line that is 20 feet wide. Each creature in that line must make a DC 32 Dexterity saving throw, taking 63 (14d8) acid damage on a failed save, or half as much damage on a successful one.

Slowing Breath(Recharge 5–6). The head exhales gas in a 180-foot cone. Each creature in that area must succeed on a DC 32 Constitution saving throw. On a failed save, the creature can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the creature can use either an action or a bonus action on its turn, but not both. These effects last for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself with a successful save.

Enervation Ray. The targeted creature must make a DC 26 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.

Ancient Gold Dragon Head

Fire Breath(Recharge 5–6). The head exhales fire in a 180-foot cone. Each creature in that area must make a DC 34 Dexterity saving throw, taking 71 (13d10) fire damage on a failed save, or half as much damage on a successful one.

Weakening Breath(Recharge 5–6). The head exhales gas in a 180-foot cone. Each creature in that area must succeed on a DC 34 Strength 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.

Telekinetic Ray. If the target is a creature, it must succeed on a DC 26 Strength saving throw or the beholder moves it up to 30 ft. in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn or until the beholder is incapacitated.

Ancient Green Dragon Head

Poison Breath (Recharge 5–6). The head exhales poisonous gas in a 180-­-foot cone. Each creature in that area must make a DC 32 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

Sleep Ray. The targeted creature must succeed on a DC 26 Wisdom saving throw or fall asleep and remain unconscious for 1 minute. The target awakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.

Ancient Red Dragon Head

Fire Breath (Recharge 5–6). The head exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.

Petrification Ray. The targeted creature must make a DC 16 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

Ancient Silver Dragon Head

Cold Breath(Recharge 5–6). The head exhales an icy blast in a 180-foot cone. Each creature in that area must make a DC 34 Constitution saving throw, taking 67 (15d8) cold damage on a failed save, or half as much damage on a successful one.

Paralyzing Breath(Recharge 5–6). The head exhales paralyzing gas in a 180-foot cone. Each creature in that area must succeed on a DC 34 Constitution saving throw or be paralyzed 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.

Disintegration Ray. If the target is a creature, it must succeed on a DC 26 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust.

Ancient White Dragon Head

Cold Breath (Recharge 5–6). The head exhales an icy blast in a 180-foot cone. Each creature in that area must make a DC 32 Constitution saving throw, taking 72 (16d8) cold damage on a failed save, or half as much damage on a successful one.

Death Ray. The targeted creature must succeed on a DC 26 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

Bite. Melee Weapon Attack: +29 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 30). Until this grapple ends, the target is restrained. Apocalypse can grapple as many targets as it has heads. After that, it can no longer bite. 

Swallow. Apocalypse makes one bite attack against a Huge or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Apocalypse, and it takes 56 (16d6) acid damage at the start of each of the Apocalypses turns.

If Apocalypse takes 100 damage or more on a single turn from a creature inside it, Apocalypse must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the tarrasque. If Apocalypse dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Frightful Presence. Each creature of Apocalypses choice within 240 feet of it and aware of it must succeed on a DC 27 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Apocalypse is within line of sight, 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 Frightful Presence for the next 24 hours.

World Shatterer Apocalypse uses all its breath attacks and eye lasers in one general direction. The range and damage of all attacks is doubled. 

Apocalypse 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 tarrasque regains spent legendary actions at the start of its turn.

Attack. Apocalypse makes one breath attack or laser attack of choice.

Move.  Apocalypse moves up to half its speed.

Chomp (Costs 2 Actions).  Apocalypse makes one bite attack or uses its Swallow.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 100 feet of the dragon must succeed on a DC 32 Dexterity saving throw or take 22 (4d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.