Spellcasting. The mage is a 9th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks). The mage has the following wizard spells prepared

Cantrips (at will): fire bolt, light, mage hand, prestidigitation

1st level (4 slots): detect magic, mage armor, magic missile, shield

2nd level (3 slots): misty step, suggestion

3rd level (3 slots): counterspell, fireball, fly

4th level (3 slots): greater invisibility, wall of fire

5th level (1 slot): dominate person

Dagger. Melee or Ranged Weapon Attack: +5 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Fireball: range 150ft 20-foot-radius sphere centered on that point must make a DC 14 Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one. The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried

Magic missile: range 120ft  instantly hits. 5 glowing darts appear and deal 3 (1d4+1) force damage.

Bonus Actions

Misty Step: Teleport up to 30ft away.

shield 

hellish rebuke

counterspell

On initiative count 20 the action to the corresponding round takes place.

1: The mage summons 1d6 animated armor minions within 5ft of itself 

2: A thunder cloud forms above, sending bolts of lightning down in 3 20ft radius cylinders on the battlefield. Any creature that starts its turn in the area of effect must make a DC 15 dexterity saving throw or take 4d6 lightning damage on a failed save, or half as much on a successful save. On each turn after this, the mage can spend a bonus action to move an area of effect up to 30ft.

3: The mage causes sturdy pillars of rock to erupt through the ground. Choose 5 points on the map to place 5ft radius cylinders of stone reaching 30ft in the air. Any creature that is in that space must make a DC 17 Strength saving throw or be knocked prone.