Amphibious. The dragon can breathe air and water.

Executioner. While in its lair, if the dragon reduces a hostile creature to 0 hit points, it immediately regains one expended use of its Legendary Actions, up to a maximum of 4 uses. If that creature is making death saving throws, the dragon immediately spends the regained Legendary Action to kill that creature.

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Acidic Eruption - Initiative Count 20:

Tell: The boss punches the ground with all its might, and cracks begin to snake outwards from the impact. Energy shines through these cracks, brightest and strongest at the epicenter of the impact, and begins growing in power. The closer a creature stands to the impact, the stronger this energy seems to be, and the more the ground cracks and trembles.

Mitigation: The players must move as far as possible from the impact to reduce the damage taken when the battlefield finally erupts.

Resolution: The ground erupts, consuming the battlefield in a blast of acid, its effects more or less lethal depending on how close a creature is to the impact. Creatures standing less than 20 feet away from the impact must make a Dexterity saving throw, taking full damage on a failure and half damage on a success. Creatures standing 25 feet or more away from the impact must make a Dexterity saving throw, taking half damage on a failure and no damage on a success. 
≤ 10ft - 10d10 - Acid Damage
≤ 20ft - 8d10 - Acid Damage
≤ 30ft - 4d10 - Acid Damage

Multiattack. The dragon makes four Rend attacks. It can replace one attack with a use of Spellcasting.

Rend. Melee Attack Roll: +16, reach 10 ft. Hit: 24 (3d8 + 10) Slashing damage plus 8 (3d8) Acid damage.

Acid Breath (Recharge 4–6). Dexterity Saving Throw: DC 22, each creature in a 120-foot-long, 15-foot-wide Line. Failure: 98 (16d10) Acid damage. Success: Half damage. This damage ignores resistance to acid. Immunity to acid causes the creature to take half damage instead of none. 

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 22, +14 to hit with spell attacks):

At Will: Detect Magic, Fear, Melf’s Acid Arrow (level 3 version)

2/Day Each: Speak with Dead, Vitriolic Sphere

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn , the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each its turns.

Cloud of Insects. Dexterity Saving Throw: DC 22, one creature the dragon can see within 120 feet. Failure: 23 (8d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Frightful Presence. The dragon uses Spellcasting to cast Fear. The dragon can’t take this action again until the start of its next turn.

Pounce. The dragon can move up to half its Speed, and it makes one Rend attack.

Rend. The dragon makes two rend attacks.