Legendary Resistance (4/Day). If Acheron fails a saving throw, it can choose to succeed instead.

Spellcasting. Acheron is over a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 26,  +16 to hit with spell attacks). Acheron has the following spells prepared:

Cantrips (at will): mage hand, prestidigitation, ray of frost, toll the dead

1st level (4 slots): detect magic, magic missile, shield, burning hands

2nd level (4 slots): hold person, invisibility, Tasha's Mind Whip, detect thoughts

3rd level (3 slots): haste, counter spell, dispel magic, fireball, slow

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

5th level (3 slots): cloud kill, scrying, hold monster

6th level (2 slots): disintegrate, globe of invulnerability

7th level (2 slots):  finger of death , plane shift, delayed blast fireball

8th level (2 slots): dominate monster, power word stun

9th level (1 slot): power word kill, time stop

Magical Resistance. Acheron has advantage on any saving throws that have to do with resisting any magical effects.

Dreadful Glare. Ranged Spell Attack:  +16 to hit, range 35 ft., one creature. The target must make a DC 26 wisdom saving throw or get 1 spell slot taken away and given to Acheron. The spell slot equals the difference between the spell save DC, and the total save of the target. For example, if the target rolled a 19 on the save, Acheron would have to take a 7th level spell slot from the target for himself. If the target does not have any spell slots that Acheron takes, the target instead takes 10d6 necrotic damage on a failed save, but none on a successful one.

Cantrip. Acheron can cast any cantrip as a bonus action on his turn.

The lich 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. The lich regains spent legendary actions at the start of its turn.

Spell. Acheron casts any spell, as long as the spell slot expended has been taken from his Dreadful Glare feature. Regardless, he may cast a cantrip in this way.

Dreadful Glare (Costs 2 Actions). Ranged Spell Attack:  +16 to hit, range 35 ft., one creature. The target must make a DC 26 wisdom saving throw or get 1 spell slot taken away and given to Acheron. The spell slot equals the difference between the spell save DC, and the total save of the target. For example, if the target rolled a 19 on the save, Acheron would have to take a 7th level spell slot from the target for himself. If the target does not have any spell slots that Acheron takes, the target instead takes 10d6 necrotic damage on a failed save, but none on a successful one.

Destroy Life (Costs 3 Actions). 1 non-undead creature within 50 feet Acheron of his choice must make a DC 20 Wisdom saving throw against his power, on a fail, Acheron fully controls the target on its next turn in combat.