Amphibious. The dragon can breathe air and water.

Control Gem. Implanted in the dragon's brain is a magic control gem. The dragon must obey whoever possesses the gem and is immune to being charmed while so controlled.

Certain spells can be used to acquire the gem. If the dragon fails its saving throw against imprisonment, the spell can transfer the gem to the spellcaster’s open hand, instead of imprisoning the dragon. A wish spell, if cast in the dragon’s presence, can be worded to acquire the gem.

A greater restoration spell cast on the dragon destroys the gem without harming the dragon.

Someone who is proficient in Wisdom (Medicine) can remove the gem from an incapacitated dragon. Each try requires 1 minute of uninterrupted work and a successful DC 20 Wisdom (Medicine) check. Each failed attempt deals 22 (4d10) psychic damage to the dragon.

The dragon is growing too powerful for the gem to control, after each time the dragon takes damage it makes a DC 20 Wisdom saving throw at disadvantage, if successful 3 time within an hour, the dragon breaks completely free of the Gem's control, rendering it inert. 

Spellcasting. The dragon is an 8th-level spellcaster. Its spellcasting ability is Charisma(spell save DC 17, +9 to hit with spell attacks). The dragon has the following sorcerer spells prepared:

Cantrips (at will): poison spray, prestidigitation, gust, message

1st level (4 slots): detect magic, magic missile, chaos bolt, fog cloud, ray of sickness

2nd level (3 slots): acid arrow, detect thoughts, see invisibility, blindness/deafness, blur

3rd level (3 slots): fear, stinking cloud

4th level (2 slots): blight, polymorph

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 9 (1d10 + 4) piercing damage plus 3 (1d6) poison damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit:  7 (1d6 + 4) slashing damage.

Poison Breath (Recharge 5–6). The dragon exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one.