Amphibious. The dragon can breathe air and water.
Legendary Resistance (4/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.
Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Mind Spike (level 5 version).
Rend. Melee Attack Roll: +15, reach 15 ft. Hit: 17 (2d8 + 8) Slashing damage plus 10 (3d6) Poison damage.
Poison Breath (Recharge 5–6). Constitution Saving Throw: DC 22, each creature in a 90-foot Cone. Failure: 77 (22d6) Poison damage. Success: Half damage.
Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 21):
At will: Detect Magic, Mind Spike (level 5 version)Mind Invasion. The dragon uses Spellcasting to cast Mind Spike (level 5 version).
Noxious Miasma. Constitution Saving Throw: DC 21, each creature in a 30-foot-radius Sphere centered on a point the dragon can see within 90 feet. Failure: 17 (5d6) Poison damage, and the target takes a -2 penalty to AC 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.
Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.