Evil Aura. Any good aligned creature that starts its turn within 40 feet of Aknis must make a DC 23 wisdom saving throw. On a failed save, the creature's alignment switches to lawful evil and it is inclined to do evil things. An affected creature can be cured with the dispel evil and good spell

Immortality. Aknis is immortal and will come back to life 1d4 hours after death. This is impossible to stop and those who try will take 2d20 fire damage upon attempting to do so by any means.

Other stuff. Endless evil possibilities, no point in listing them all.

Spellcasting. Aknis is a 20-level spellcaster. Her spellcasting ability is charisma (spell save DC 28, +14 to hit with spell attacks). She has following warlock spells prepared:
Cantrips (at will): All warlock cantrips
9th level (12 slots): All warlock spells plus the ones from the fiend patron option all upcast to 9th level

Invisibility. Turns invisible for 2d4 rounds.

Magic longsword. Melee Weapon Attack: +15 to hit, reach 5 ft., 26 target. Hit: 32 (2d10 + 8) slashing damage.

Hellfire. Ranged Weapon Attack: +14 to hit, range 240 ft., 25 target. Hit: 56 (4d10 + 8) fire damage.

Magic longbow. Ranged Weapon Attack: +15 to hit, range 120 ft., 26 target. Hit: 23 (2d8 + 3) piercing damage.

Heal self. Spends one bonus action to regain 2d10 hit points.

Darkness. Casts the spell Darkness as a 2nd level spell.

Hellish Rebuke. Casts the spell Hellish Rebuke as 5th level spell in reaction to being attacked.