Max 1 attack damage

A 17+ level wizard, sorcerer, bard, or arcana domain cleric can cast meteor swarm, dealing an average of 140 damage. Thats a lot. It's enough to kill a lich, not taking into account legendary resistances. However, a 20th level zealous rougelock with the right feats, items, and allies, can deal a lot more.

The build

Your level and feat makeup for this build will look like this.
Race; Bugbear
Class; Assassin Rouge 3, Hexblade Warlock 9, Tempest cleric 8
Feats; You need the martial adept, orcish fury, charger, and fighting initiate : dueling feats. Due to all theese feats your scores will be horrible, but that doesn't matter, your about to spend 700 years in a hole reading manuals of quickness of action!
Allies valor bard 17, grave cleric 2.
Items
Vorpal sword
Oil of sharpness
Purple worm poison
Eldritch Claw tattoo
Blood fury tattoo
Mill road murderers tooth of dahlver-nar

The prep

1. Receive inspiration from your valor bard
2. take an vorpal scimitar with elemental weapon cast on it from your bard friend
3. Activate a glyph of warding with enlarge/reduce, enlarging yourself
5. Have your grave cleric use path to the grave on your foe
6. Cast bestow curse on your foe.
6. Shocking grasp yourself and cast absorb elemental as a 9th level spell from a scroll
7. Ambush your foe

The attack

1d6 from your scimitar
+5d10 Banishing Smite
+5d8 eldritch smite
+1d4 Elemental weapon
+1d6 Maneuver
+1d4 Enlarge
+2d6 surprise attack
+1d6 Orcish fury
+12d6 Purple wurm poison
+1d6 Eldritch Claw tattoo
+1d8 bestow curse
+4d6 blood fury tattoo
+3d10 mill road murderers tooth of dahlver-nar
+6 Hexblade’s curse
+3 Magic weapon
+3 Oil of sharpness
+10 Dex 30
+5 Charger feat
+108 Absorb elements (you deal lightning damage, and max that with channel divinity.
+Booming blade +112 (you deal thunder damage damage. Because you are making one attack, you can max the immediate damage and absorb elements with one use of channel divinity. You need your second channel divinty to max the follow up dice when they move, which they do because thunderbolt strike)
+36 Divine strike (damage is maxed)
+2 dueling

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This brings average total damage to 582, which is doubled to 1164 due to path to the grave, and your max total to 1636. For reference, 1164 damage is enough to kill 8.62222... lichs if your DM uses cleave rules.

ways to deal more damage

There are ways that I know of to deal more then 1364 damage, and likely several thing I do not know of to boost damage even further.

Way 1: moonblade

The moonblade is a magic weapon that gains a new rune for every user it's had. One of these runes says

When you hit with an attack using the moon blade, the attack deals an extra 1d6 slashing damage.

This effect stacks, and there is no limit to the number of runes on the moonblade. With infinite runes, you could deal infinite damage.

Way 2: Whirlwind attack

A vorpal greatsword wielding hunter ranger 11 paladin 2 hexblade warlock 7 with whirlwind attack, boots of flying, and many of the things used on the War-gue-lic surrounded by 448 quasits all of which have taken 1 damage (for colossus slayer) and are effected by paths to the grave can deal 394,240 (2920.296296 lichs) damage in one attack, but to 448 different creatures (you will deal 880 damage to each quasit, or just over 6.5 lichs)