Mr. bones is a 20th-level spellcaster. their spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The lich has the following wizard spells prepared:

Cantrips (at will): Mage Hand, Prestidigitation, Ray of Frost

1st level (4 slots): Detect Magic, Magic Missile, Shield, Thunderwave

2nd level (3 slots): Melfs Acid Arrow, Detect Thoughts, Invisibility, Mirror Image

3rd level (3 slots): Animate Dead, Counterspell, Dispel Magic, Fireball

4th level (3 slots): Blight, Dimension Door

5th level (3 slots): Cloudkill, Scrying

6th level (2 slot): Disintegrate, Globe of Invulnerability

7th level (2 slot): Finger of Death, Plane Shift

8th level (1 slot): Dominate Monster, Power Word Stun

9th level (1 slot): Power Word Kill, Wish

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