Augustus the Prisoner

Legendary Resistance (3/Day): If Augustus fails a saving throw, it can choose to succeed instead.

Grimoire Infinitus. Augustus holds this grimoire that is a culmination of all of his knowledge and grants special properties according to the competence of the wielder.

Augustus is attuned the following magic items:

• Black Robe of the Archmagi
• Staff of the Magi

Innate Spellcasting. Augustus' innate spellcasting ability is Intelligence (spell save DC 20, +14 to hit with spell attacks). It can innately cast the following spells requiring no material components:

At will: Alarm, Detect Magic

3/day each: Misty Step, Counterspell

1/day each: Suggestion, Dominate Person, Planar Binding

Spellcasting. Augustus is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +14 to hit with spell attacks). Augustus has following spells prepared:
Cantrips (at will): Control Flames, Fire Bolt, Mage Hand, Minor Illusion, Shocking Grasp 
1st level (4 slots): Shield, Sleep, Thunderwave

2nd level (3 slots): Flaming Sphere, Hold Person*, Shatter

3rd level (3 slots): Fireball, Magic Circle, Slow

4th level (3 slots): Confusion, Stoneskin*, Private Sanctum, Wall of Fire

5th level (3 slots): Immolation, Telekinesis, Wall of Force

6th level (2 slots): Circle of Death, Globe of Invulnerability, Instant Summons

7th level (2 slots): Delayed Blast Fireball, Forcecage

8th level (1 slot): Dominate Monster, Maze

9th level (1 slot): Prismatic Wall

Augustus casts spells marked with a * before combat starts.

Quarterstaff. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 2) bludgeoning damage.

Augustus the Imprisoned can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Augustus regains spent legendary actions at the start of their turn.

Detect. Augustus makes a Wisdom (Perception) check.

Fireball. Augustus casts Fireball.

Suggestion (2 Actions). Augustus casts Suggestion