Clockwork Lich, Archmagus of the Gears

Immutable Form. The clockwork lich is immune to any spell or effect that would alter its form.

Magic Resistance. The clockwork lich has advantage on saving throws against spells and other magical effects.

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

Rejuvenation. If the clockwork lich is destroyed, its body reforms in 1d10 days in its lair. Only the application of holy water, dispel magic, or remove curse during that time prevents the rejuvenation.

Multiattack. The clockwork lich makes three attacks: one with its steam-powered staff, one with its clockwork blade, and one with its arcane-infused touch.

Steam-Powered Staff. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 22 (2d10 + 4) bludgeoning damage plus 21 (6d6) fire damage.

Clockwork Blade. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (1d10 + 4) slashing damage plus 17 (5d6) lightning damage.

Arcane-Infused Touch. Melee Spell Attack: +13 to hit, reach 5 ft., one target. Hit: 38 (9d8) force damage. The target must succeed on a DC 21 Constitution saving throw or be stunned until the end of its next turn. 

Legendary Actions The clockwork lich can take three 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 clockwork lich regains spent legendary actions at the start of its turn.

• Clockwork Blades. The lich makes a clockwork blade attack.
• Arcane Overload (Costs 3 Actions). The lich unleashes a surge of raw arcane energy in a 40-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 66 (12d10) force damage on a failed save, or half as much damage on a successful one.