Amphibious. The dragon mage can breathe air and water.

Spellcasting. The dragon mage is a 10th level spellcaster, it's spellcasting ability is Intelligence (DC 15, +7 to hit with spell attacks). It has the following wizard spells prepared. 

Cantrips (at will): Blade ward, poison spray, minor illusion, prestidigitation, shocking grasp 

1st level (4 slots): Chromatic orb, fog cloud, mage armor, ray of sickness

2nd level (3 slots): Detect thoughts, ray of enfeeblement, suggestion

3rd level (3 slots): Animate dead, counterspell, clairvoyance 

4th level (3 slots): Conjure minor elementals, polymorph

5th level (2 slots): cloudkill

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

Arcane Siphon (recharge 6). The dragon mage targets up to 3 creatures that are able to cast spells that it can see within 30 feet of it. Those creatures must make a contest using their spellcasting ability against the dragon mage's spellcasting ability. On a fail, the creatures loses the highest spell slot available to a maximum of 5th level and the dragon regains spells slots equal to what the creatures lost. The dragon mage cannot gain more spell slots than it can have in this way.

Bite. Melee Weapon Attack: +7  to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 7 (2d6) poison damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 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 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.