Just seems like something fun to do away from the regular melee. I had thought about running a sorcerer with either a multiclass into paladin for smite and stretched for the extra attack or like fighter for something like the champion subclass and maybe for the extra attack too. I didn’t know if people have made this build before so just looking for some tips on what to look for. 

Flame Blade is a Druids only spell so you would need at least three levels of Druid.

Probably the most common build is an Arcane Trickster Rogue using Shadow Blade and Booming Blade and Sneak Attack all at the same time with no need to multiclass. Alternatively you could do it with an Eldritch Knight fighter instead and take advantage of the Extra Attacks (up to 4 attacks/turn at level 20) instead of Sneak Attack.

I didn’t think about eldrich or arcane trickster. 

I was also looking at the Class features variant that gave the sorcerer flame blade.  

Quote from OG_Diddles >>

I didn’t think about eldrich or arcane trickster. 

I was also looking at the Class features variant that gave the sorcerer flame blade.  

 Okay. But Sorcerers suck at Melee combat for the most part since they get no armor and very few HP. In fact, most Sorcerers try to stay as far away from melee combat as possible, that’s why the Distant Spell Metamagic option is so popular. Just food for thought.

It's worth noting that the actual Flame Blade spell itself largely sucks donkey rocks.  It's almost strictly inferior to Dragon's Breath; it deals very little damage per swing and you give up your entire action to make one swing with it.  It's pretty dooky.

That said, if you really want to use it?  Try and convince your DM to allow you to rewrite the spell to make it similar to Shadow Blade, i.e. "you create a sword made of flames in your hand.  it deals 3d6 fire damage, but otherwise works like a regular sword".  if you can get away with that, I'd honestly recommend you look into bard.  Magical Secrets means you can take whatever spells you like, though only Lore gets it early.  Nevertheless, you can do a Swords or Valor bard who gets basic combat bonuses like an extra attack as well as basic armor and weapon proficiencies.  

if you have to use Flame Blade as written, then yeah.  Dump Flame Blade and go for Shadow Blade.  Shadow Blade rogues are a thing, and a Shadow Sorcerer with a shadow blade can occasionally get work done with its funky Darkness casts.  it's not good, but it's not the literal worst, either.

Bard would be good too. But they can still be squishy in melee if not done right so they’re less forgiving of mistakes on the melee front than Fighter or Rogue.

But, Bards are way cooler and can do more and more interesting stuff IMHO. So you can probably not go wrong with the bard, but make sure to tailor your spell selection so that your Magic doesn’t end up becoming more useful than your melee or you can accidentally sabotage your own build. Bards are so good that’s actually not hard to do, so you will just have to keep that in mind. That’s a nonissue for Fighter/Rogue since they already have that design focus built in. So the choice might come down to how much attention you want to have to play to your level-up choices.

You might also want to look at Hexbladeadin. I know Yurei will probably die a little inside when they see me suggest that, but it is functionally a strong Melee build for Shadow Blade. 

The character I made for Shadow Blade was a Bladesinging Wizard. They can get a lot of AC but they're still super squishy when they do get hit - and they only get 2 attacks.

Two attacks is the most anybody gets unless they're a fighter.

Another neat trick is an Artificer artillerist. You need a way to get a bonus cantrip (race bonus, feat or multiclass), but then you can do a booming blade + arcane firearm bonus as an action+ eldritch canon with a push effect as bonus action. 

One option might be Battle Smith 5 / Wizard x. 

This gets you two attacks per round and SAD on Intelligence. 

Nm

One build that I created was a Paladin/Sorcerer Multiclass. This build is mainly made for late game tiers of play since it takes a while to build up. 

Going Paladin 6 / Sorcerer 14 will allow you to do many things, but to answer your question, it will give you Shadow Blade at level 9 (Paladin 6/Sorcerer 3). You then have extra attack from Paladin to attack with. If you want to bump up the damage even more, use the attack action to attack twice and then quicken spell (meta magic) Green Flame Blade for a third attack and then add a smite. 

As this build levels up you have access to tons of spell slots to smite or up-cast shadow blade with. In addition you have more sorcery points to quick spell. 

I'm playing a Fighter 1 Celestial Warlock 6 right now, and he relies heavily on the Shadow Blade + Green Flame Blade combo.  It's a pretty powerful strategy at almost every level.  It kind of tails off a bit after you get to about level 12 or so in Warlock, but up until that point it's quite powerful.

At level 1 you are a standard fighter with the defense fighting style, and you can already start your Sword & Board tactics at this level and have an excellent AC.

At level 2 you get to add Green Flame Blade and Sacred Flame to your arsenal, you get 2D6 Healing Light per long rest, you still have Second Wind once per short rest from your first level in Fighter, and you can cast Hex once per short rest to get a nice damage boost.  Because you have Second Wind, the defense fighting style, and a shield, you're much tankier than most Warlocks.  You've also got proficiency in Constitution Saving throws, which comes in very handy when maintaining concentration on a spell.

At level 3, you get a 2nd spell slot, and you can take the Fiendish Vigor invocation which is going to be overpowered for a few levels before it quickly tails off, and you get one other invocation.  At level 3, starting every combat with 8 temporary hit points is huge.  You don't even have to do a short rest to get these 8 temporary hit points at a time when your character should have just 26 or 29 max HP depending on your Constitution bonus.

At level 4, you get access to Shadow Blade, and when you take Pact of the Chain, you get an invisible familiar and access to the Gift of the Ever Living Ones to turn your Second Wind into an automatic heal 11 every short rest (for a Bonus Action), and your Healing Light D6s (4 of them now) heal you for 6 automatically (also for a Bonus Action)

At level 5, you can take the war caster feat to go with your proficiency in constitution saving throws to make sure that you can maintain concentration on Shadow Blade.  Plus, your Green Flame Blade gets a damage boost of 1D8 on both the primary target and the secondary target (if there is one).

At level 6, your Shadow Blade adds another D8 of damage, so it's up to 3D8 plus strength or dexterity now, and you get to choose one more invocation. You can take the Tomb of Levistus invocation to make your character even more tanky.  You're always adding 1 more D6 of Healing Light at every level, and your Tomb of Levistus will also gain 10 HP at every level.

At level 7, you get to add your Charisma modifier to your Green Flame Blade damage (and to your Sacred Flame damage).  And you get resistance to Radiant Damage (it's not likely to come up very often unfortunately).

At level 8, you get to choose between Shadow Blade for more damage, or Shadow of Moil to be even more difficult to hit.  And you get another invocation.

At level 9, your proficiency bonus goes up, and you get an ASI or a feat.  It's a boring level, but it's still helpful.

At level 10, your Shadow Blade adds another D8 of damage, so that it now deals 4D8 damage (plus strength or dexterity), and you can add Synaptic Static for when you want a fireball type spell that also debuffs enemies.  And you get another invocation.

At level 11, your Green Flame Blade adds another D8 of damage, so that it now deals 2D8+charisma modifier fire damage on top of the 4D8+ strength or dexterity psychic damage of Shadow Blade.  So that's 6D8 + charisma + strength/dexterity total damage.  Plus another 2D8+charisma modifier to an adjacent enemy if there is one.  You also get 10+ Charisma modifier temporary hit points at the end of every short rest (which as a Warlock, hopefully you're able to do a couple of times per day), and your party gets 5+charisma modifier temporary hit points at the same time.  These are going to scale up by 1 HP every level for you and 1 HP every other level for your allies.  Your self healing capabilities with Second Wind and 11D6 of Healing Light (both of which use Bonus Actions) are quite good, and you have a nice 100 temporary hit points from Tomb of Levistus that last for one round, and you can use this ability once per short rest.

At level 12, you get a 3rd spell slot and a Mystic Arcanum. It's around this level that the strategy starts to tail off a bit.

At level 13, you get an ASI or a feat, along with your proficiency bonus going up, and you get an invocation.  

At level 14, you get a Mystic Arcanum

At level 15, you get a nice Celestial capstone ability of Searing Vengeance, where once per day when you're about to make a death saving throw, you can instead jump to your feet, heal back up to half your max HP, deal 2D8+charisma radiant damage to all enemies within 30ft, and briefly blind them (with them not even getting a saving throw against the effect)

At level 16, you get access to a couple of nice invocations, being able to cast Hold Monster at will against a limited number of enemy types that are somewhat common at higher levels (although you can only cast it on a specific creature once per day with the ability), and you can choose an invocation that lets you cast invisibility at will. You also get a Mystic Arcanum, which gives you access to [spell[Feeblemind[/spell]

At level 17, you get an ASI or a feat, your proficiency bonus goes up, and your Green Flame Blade gets better.

At level 18, you get a 4th spell slot, and you get your last Mystic Arcanum, which gives you access to True Polymorph

At levels 19 and 20, you should probably take two more levels in Fighter for Action Surge and an archetype, or two levels in Rogue for Expertise, Sneak Attack (why not?), and Cunning Action.


So that's how I designed my tanky Warlock that relies heavily on Shadow Blade.

finally. play the variant sorcerer (with flame blade) pick up shadow blade and then multi class paladin. it is not as good as a properly built sorcadin, but it should do the trick

Quote from Emmber >>

The character I made for Shadow Blade was a Bladesinging Wizard. They can get a lot of AC but they're still super squishy when they do get hit - and they only get 2 attacks.

 this is soooo much fun. this was the only character I played until lvl 20 and it was bonkers. I threw in two levels of paladin (ik I put paladin in everything) and that let him run around with god tier ac smiting everything. I had foresight and elven accuracy. my god. the nostalgia is killing me. i might have to play this again

Both flame blade and shadow blade are concentration spells, no?  That sort of puts the kibosh on this unless your DM is willing to let you work around.  It is a fun idea though.  I actually have a Pact of the Blade Warlock right now who fights with a flame tongue short sword and a shadow blade in his off hand.  Both are light, finesse weapons (for two attacks per round) and the thirsting blade eldritch invocation gives me an additional attack.  The other bonus is that as a Warlock, you are always casting shadow blade at the highest level you've got.  Nasty.