Amphibious. The dragon can breathe air and water.

Adaptive Magical Resistance. This creature's scales seem to tremble in the presence of magic as if drawn by the source. The creature's scales turn the color of the last magic cast on it and gain resistance to that type of magic. The resistance lasts 1 minute or until another type of spell damage replaces it.

Adaptive Physical Resistance. The creature's scales twist and round out when attacks are thrown at it. The creature's scales change to give resistance to the last type of physical attack towards it. The resistance lasts 1 minute or until another type of physical attack replaces it.

Legendary Resistances (1/day): If this creature fails a saving throw it can use a Legendary resistance to succeed. 

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: 15 (2d10 + 4) piercing damage plus 7 (2d6) damage currently magically resistant to (if there is no current magic resistance the type is necrotic) damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) (Bludgeoning, Piercing, Slashing) Whatever physical damage it is currently resistant to.

Changing Breath (Recharge 5–6). The dragon exhales the type of damage currently magically resistant to (if there is no current magic resistance the type is necrotic) in a 60-foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking (8d8) lightning damage on a failed to save, or half as much damage on a successful one