Acid Absorbtion. When the dragon is subjected to acid damage, it takes no damage and instead regains an amount of hit points equal to the acid damage.

Amphibious. The dragon can breathe air and water.

Colossal. The dragon is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever the dragon makes a weapon attack, regardless of whether the attack hits or misses, each creature within 10 feet of the attack's target other than the target or the dragon itself must succeed on a DC 25 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The dragon's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Corrosive Touch. If the dragon hits a creature that is wearing nonmagical armor or wielding a nonmagical shield with a weapon attack, the armor or shield corrodes and is destroyed. Similarly, if the dragon is hit by a nonmagical weapon, the weapon is destroyed immediately after dealing damage.

If the dragon instead hits a creature that is wearing no armor and wielding no shield, with a weapon attack, the creature must succeed on a DC 25 Constitution saving throw or takes an additional 14 (3d8) acid damage and suffers disadvantage on its attack rolls, ability checks and saving throws until the end of its next turn.

In addition, a creature takes 27 (6d8) acid damage if it touches the dragon or starts its turn while in contact with  the dragon, such as when it is grappled by it.

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

Revelation in Cruelty. If the dragon reduces a creature to 0 HP with an attack or scores a critical hit, it has advantage on all attack rolls and ability checks until the end of its next turn. 

Spellcasting. The dragon is a 20th level spellcaster. It's spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). It can cast the following spells. requiring no material components:

• Cantrips (at will): acid splash, chill touch, mage hand, prestidigitation, thaumaturgy
• 1st level (4 slots): detect magic, identify, Tasha's hideous laughter, Tasha's caustic brew, shield
• 2nd level (3 slots): spike growth, cloud of daggers, misty step
• 3rd level (3 slots): bestow curse, hunger of hadar, remove curse
• 4th level (3 slots): death ward, elemental bane, Evard's black tentacles
• 5th level (3 slots): summon draconic spirit, legend lore, scrying
• 6th level (2 slots): harm, primordial ward
• 7th level (2 slots): forcecage, divine word
• 8th level (1 slot): Abi-Dalzim's horrid wilting, power word pain
• 9th level (1 slot): time ravage

Treasure Sense. The dragon knows the location of all magic items and of accumulations of mundane treasure within 5 miles of it.

Multiattack. The dragon can use its World-Ending Presence, and if it was flying at the start of its turn, it can use its Crushing Landing. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 22 (2d12 + 9) piercing damage plus 14 (3d8) acid damage and the dragon can grapple the target as part of that attack.

Claw. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) slashing damage plus 14 (3d8) acid damage. If the target is a creature, it must succeed on a DC 25 strength saving throw or be flung up to 3d4 × 10 feet away from the dragon, landing prone.

Tail. Melee Weapon Attack: +17 to hit, reach 35 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage plus 14 (3d8) acid damage and the target must make a DC 25 Strength saving throw or be knocked prone.

World-Ending Presence. Each creature of the dragon's choice that is within 300 feet of the dragon and aware of it must succeed on a DC 21 Wisdom saving throw or drop whatever it was holding and become frightened for 1 minute. A frightened creature must use the dash action on each of its turns, moving as far away from the dragon as it can, using the safest possible route.  If a creature ends its turn in a loaction where it does not have line of sight to the dragon, it can repeat the saving throw, 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 World-Ending Presence for the next 24 hours.

Crushing Landing. If the dragon is flying, it can attempt to land forcefully in a space of its choice, even if that space is occupied by other creatures. When it does so, its movement speed is reduced to 0 until the end of the turn, and each creature in the dragon's space or within 30 feet of it, other than the dragon itself, must make a DC 25 Dexterity saving throw or take 26 (4d12) bludgeoning damage plus 14 (3d8) acid damage and be knocked prone.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 500-foot line that is 20 feet wide. Each creature in that area must make a DC 25 Dexterity saving throw, taking 81 (18d8) acid damage on a failed save, or half as much damage on a successful one. In addition, a creature who fails the saving throw has disadvantage on attack rolls, ability checks and saving throws until the end of its next turn.

Quicken Spell (3/day). The dragon casts one of its spells of 5th level or lower with a casting time of 1 action.

Sadistic Magic. If the dragon can see a creature within 120 feet of it casting a spell or using a magical ability to restore hit points to a creature, it can use its reaction to pervert that magic. If the dragon does so, the target of the healing spell or ability does not regain hit points and instead takes necrotic damage equalt to the amount of healing it was supposed to receive. 

The dragon 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 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 30 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 18 (2d8 + 9) bludgeoning damage plus 14 (3d8) acid damage and be knocked prone. The dragon can then fly up to half its flying speed.

Cast a Spell (Costs 2 Actions). The dragon casts one of its spells of 5th level or lower.

Acid Rain (Costs 3 Actions). The dragon causes acid to rain onto all creatures within 300 feet of it. Each creature within that area must succeed on a DC 25 dexterity saving throw, taking 27 (6d8) acid damage or half as much on a success. Due to the many puddles of acid on the ground, the ground in this area counts as difficult terrain until the end of the dragon's next turn and a creature takes 5 (1d8) acid damage for every 5 feet it travels.