Spellcasting. The mage is a 9th-level spellcaster. Its spellcasting ability is Charisma  (spell save DC 13, +5 to hit with spell attacks). The mage has the following wizard spells prepared:

Cantrips (at will): eldritch blast, light, mage hand, message

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

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

3rd level (3 slots): counterspell, fireball, vampiric touch

4th level (3 slots): blight, dimension door

5th level (1 slot): scrying

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.

Bonus Action

Summon Hellhound. Summon a devilish mastiff up to 30ft away.

Villain Actions

These are used in order

Daggaz Rune. Mark on a surface the Daggaz Rune. Looks like two parallel lines with a cross connecting them. The rune burns red when finished, and makes a duplicate underneath one target within 60ft. DEX Save 14 dealing 3d6 fire damage. On a successful save take half damage. As a gout of flame bursts up from the copy of the rune.

Algiz Rune. Mark on a surface the Daggaz Rune. Looks like a Y but with the central line continuing upwards. The rune burns white when finished, and makes a duplicate underneath one target within 60ft. DEX Save 14 dealing 3d6 cold damage. On a successful save take half damage. As a geyser of icy wind bursts up from the copy of the rune.

Uruz Rune. Mark on a surface the Daggaz Rune. Looks like a strange lower-case N. The rune bubbles purple when finished, and makes a duplicate underneath one target within 60ft. DEX Save 14 dealing 2d6 necrotic damage. On a successful save take half damage. As a funnel of dark energy bursts up from underneath.

The Summoning. Yannik begins chanting, and floats high into the air, to the central point between the runes. He slices open his hand, digs a finger into midair, and begins pulling open the very fabric of reality, revealing a dark, burning red visage awaiting just behind it. As each player ends their turn, Yannik will pull it open further, revealing more of the Balor he is summoning. After 5 turns including his own which he cannot use, the Balor is summoned through the rift.