I wanted to share this build and get feedback. The design is a critical hit maximal damage build, but features heavy skill proficiencies and expertise. Compare it to the self-proclaimed leading build below, and I think you'll find that it is better for max damage. Also, if paired with a teammate, can be insanely devastating.
Here is the "leading" build for reference. The author even admits this build is more damaging: https://www.patreon.com/posts/broken-one-most-21808193
THE ROLE PLAY
So, this character is titled the "Paladin of Mystery."
The character starts as any rogue with any backstory. The character concept is developed through the first nine levels of gameplay. The character starts as a rogue but after two levels, repents a life of burglary and takes two levels of paladin. This paladin will not pick up an oath, so the oath for flavor is that the paladin begins to train in service to the god of the arcane.
However, the paladin realizes before taking the third level oath that in order to truly protect the god of the arcane, the character must "know thy enemy" and learn about the tactics of would-be assassins against the god. The character takes level three of rogue, picks up assassin from PHB, and takes a feat at level four rogue.
The character is best played as a Half-Elf, starting stats Strength 13, Dex 16, and Charisma 17. One remaining stat will have to be a weak stat at 9 (-1). This allows for Elven Accuracy (XGE) to be the feat at level six.
At this point, the character decides that service to the glory of the god of the arcane will be most effective through song and performance, rallying patronage of the god and inspiring the masses. The character takes level six in bard, resulting in 2 Lvl Paladin, 4 Lvl Rogue Assassin, and 1st Lvl Bard. At 3rd Lvl Bard, the character selects College of Whispers from XGE.
The character is now essentially a secret agent in service to the god of the arcane - An assassination, espionage, propaganda expert. Does this put the Paladin on the evil alignment spectrum? My Paladin of Mystery is lawful good, but uses these functions in service to god of the arcane for a better world. Hypocrisy?
You could be extra mysterious and from level two onward tell your party that you are a Lvl 2 Rogue/ Lvl X Paladin, choosing a cover-story Oath for flavor. This allows you to flavor the reputation of your character in any given game setting. Feeling nature loving? Say you are Oath of Ancients. Feel like avenging a cruel act against your god? Say you are Oath of Vengence. There are a lot of flavor opportunities.
THE GAME PLAY
The key mechanics of this build are the combination of Assassinate, Expert Deception, Mantle of Whispers, Magic Secrets, Psychic Blades and to a lesser extent Sneak Attack. There is another Bard Paladin build that breaks the game with extra-planar glyph play, but this build does not directly go that route. If you call out dibs on a portable hole or bag of holding early and acquire one, you can still use glyph play with this build.
"Surprise" rules state that surprise is a condition when a hostile does not know of a threat. Here is the leap of faith - The deception vs. insight element of Mantle will have to qualify as the lack-of-awareness precondition to satisfy the prerequisites for surprised condition. This way, if the BBEG has any hench-lings, casting Mantle after killing an underling allows you to walk right up to BBEG and critical hit.
Combine that crit with Psychic Blades, Smite, and Sneak Attack and bring the pain.
With your first Magic Secrets you can pre-cast Shadow Blade and get that base damage as high as possible.
At your first Magic Secrets, you could also have the opportunity to spend an hour and glyph T's Transformation anywhere within 10 minutes of the BBEG adding advantage, damage, and an extra attack for another smite. The glyph play allows you to stack T.T.'s with Shadow Blade. Well worth the cast time vulnerability. A wise DM can make this difficult.
If you get a bag of holding or portable hole, you can climb into the extra-planar dimension inside those objects and store your glyphs there, so the one hour wait reduces to a few turns. This method also allows you to stack Searing Smite at as high of a spell level you can cast. Ouch!
THE TEAM PLAY
Now for the stupidity. You can enlist a newer player to easily do this. The only cheese mechanic here is that you have to get pre-approval that your character may drop their weapon as a free action, and drop it off to their side halfway between them and another player. Also, you have to get the approval that a character can pick up the item from halfway between the two players. There aren't great rules on the mechanic of dropping and picking up items, so here is another thread that discusses:
It is a stretch, but if you can casually get pre-approval, the method will work.
The second character should be a Lightfoot Halfling Lvl 3 Rogue Assassin/ Lvl X Fighter Champion. The second character has to roll earlier than the paladin in the order of initiative.
So, as the Paladin takes a deception vs. insight check for Mantle surprise, the Halfling uses Naturally Stealthy to remain obscured behind the Paladin.
Round 1, Halfling: Moves forward and diagonally through the Paladin's square. Ready action: "When the Paladin drops the Shadow Blade, I pick it up and Attack Action the BBEG."
Round 1, Paladin: Crit Nova on BBEG, free action drops Shadow Blade half-way between Paladin and Halfling.
Reaction, Halfling: Free action picks up Shadow Blade, and attack actions BBEG with as many extra attacks as allowed.
Round 1, BBEG: Surprised.
Round 1, Remaining PCs: Enter the room and unleash the beast.
Round 2, Halfling: Shadow Blade disappears. Free action, draw weapon. Action surge with extra attacks.
Round 2, Paladin: Bonus Action, summon Shadow Blade. Attack action.
Round 2, BBEG: Makes first attack.
Round 2, Remaining PCs: Continue destruction.
BBEG should be dead at this point. The end.
EDIT 1: It may be an exploit to break the weapon drop rules to allow the halfling shadow blade shanking action. TEAM PLAY could be a party centered around the 3 level Assassin dip: lightfoot halfling fighter/druid, paladin of mystery, half-orc ranger, and non-assassin paladin 2/ bard X as caster. Have fun!
EDIT 2: Took out some copyright issue stuff. Also, here have this:
15. COPYRIGHT NOTICE
Open Game License v 1.0a Copyright 2000, Wizards of the Coast, LLC.
System Reference Document 5.1 Copyright 2016, Wizards of the Coast, Inc.; Authors Mike Mearls, Jeremy Crawford, Chris Perkins, Rodney Thompson, Peter Lee, James Wyatt, Robert J. Schwalb, Bruce R. Cordell, Chris Sims, and Steve Townshend, based on original material by E. Gary Gygax and Dave Arneson.
Any other reference that may exist to Wizards of the Coast LLC materials is claimed free as fair-use according to a commentary/ satire exclusion.
EDIT 3: Notes on alignment.