Amphibious. The dragon can breathe air and water.

Spellcasting. The dragon is a 9th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). The dragon has the following spells prepared:

Cantrips (at will): minor illusion

1st level (4 slot): silent image, cure wounds

2nd level (4 slot): invisibility

3rd level (3 slots): major image 

4th Level (2 slots): polymorph

5th Level (1 slot): mislead

Shapechanger. The dragon can use its action to polymorph into a Small or Medium creature it has seen, or back into its true form. Its statistics, other than its intelligence, match those of the new form. It retains its spellcasting abilities while in its new form so long as spellcasting is possible for that form. Any equipment it is wearing or carrying isn't transformed. The dragon assumes the hit points of its new form. When it reverts to its normal form, the dragon returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious.

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

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 7 (2d6) poison damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) 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 15 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.