Clockwork Lich

Legendary Resistance (4/Day, or 5/Day in Lair). If the lich fails a saving throw, it can choose to succeed instead. 

Clockwork Heart. The lich is kept alive by its clockwork heart. If the lich is reduced to 0 HP, the heart dislodges itself and attempts to flee. The heart has all the lich's game statistics, but it's creature type is construct, its size is tiny, its AC is 15, its HP is 25, its strength score is 6, and it can only take the disengage, dash, or hide action. (It can still take all of its old reactions and bonus actions.) If the heart is not destroyed, it builds the lich a new body, reviving it in 48 hours with all its hit points.

Dark Genius. The lich can use its intelligence modifier in place of any check that would use wisdom or charisma

Multiattack. The lich makes three attacks, using Robotic Parasite or Paralyzing Agent in any combination.

Robotic Parasite. Ranged Attack Roll: +13 to hit, Range 60 ft. Hit: 2 (1d4) piercing damage and 36 (6d8+12) necrotic damage. At the start of it's next turn, the lich regains HP equal to half the amount of necrotic damage dealt.

Paralyzing Agent. Melee Attack Roll: +12 to hit, reach 5 ft. Hit: 15 (3d6 + 5) Poison damage, and the target has the Paralyzed condition until the start of the lich’s next turn.

Spellcasting. The lich casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 20):

At Will: Detect Magic, Heat Metal, Dispel Magic, Cloudkill (level 5 version), Invisibility, Elemental Bane (level 5 version), Mage Hand, Prestidigitation, Burning Hands (level 5 version)
2/Day Each: Summon Construct, Dimension Door, Bigby's Hand
1/Day Each: Chain Lightning, Gate, Blade of Disaster, Feeblemind

Eldritch Cannon: The Lich creates a medium turret in an unoccupied space within 5 ft. The turret is a magical object with AC 18 and 100 HP. It has immunity to psychic and poison damage, and acts immediately after the lich's turn, making a ranged attack roll against a creature hostile to the lich within 120 ft of it, using the lich's spell attack modifier. on a hit it deals 16 (4d8) necrotic damage and pushes the target up to 15 ft. away from it. if the cannon is reduced to 0 HP, it explodes in a 10 radius sphere, dealing 20 (4d10) force damage and 20 (4d10) necrotic damage. The lich can only have one cannon at a time. if the lich attempts to create a new cannon, the old one is destroyed.

Protective Magic. The lich casts Counterspell or Shield in response to the spell’s trigger, using the same spellcasting ability as Spellcasting.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the lich can expend a use to take one of the following actions. The lich regains all expended uses at the start of each of its turns.

Life Transfer: The lich places a tiny health sapping device on a creature it can see within 60 ft. That creature takes 9 (3d6) necrotic damage. The lich regains HP equal to the amount of damage dealt. The targeted creature continues to take damage in this way until they or another creature takes an action to remove the device. If the targeted creature regains HP while the device is attached to them, they do not regain HP, and the lich regains HP equal to the amount they were meant to regain instead. The lich can only have one life transfer device attached to a creature at a time, and can transfer a previously existing one to another creature in range with this action.

Disrupt Heart. Constitution Saving Throw: DC 20, each creature that isn’t an Undead or Construct in a 20-foot Emanation originating from the lich. Failure: 31 (9d6) lightning damage. Success: Half damage. Failure or Success: The lich can’t take this action again until the start of its next turn.

Unfathomable Calculation. The lich casts foresight on itself, using the same spellcasting ability as Spellcasting. The spell ends at the start of it's next turn, instead of the usual duration. The lich can’t take this action again until the end of its next turn.