The bonus action attack from two-weapon fighting doesn't add the attacker's ability modifier to damage without the two-weapon fighting fighting style, which a 3rd-level warlock doesn't have. Additionally, the shadow blade uses strength or dexterity for its attack and damage rolls, not charisma. Also, a longsword is just 1d8, not 1d8+1. If you've given them a +1 magic weapon at 3rd-level, well... that's a thing you did.

But yeah, for the cost of a 2nd-level spell slot, this character could do between 11 and 32 damage per round by the second round of combat.

[EDIT] It would be by the third round, because Hexblade's Curse is also a bonus action. Most combats don't go much more than three rounds as it is.

Hex warrior say you touch one weapon and that uses CHA bonus. I assume that’s the pact weapon. The shadow blade would use dex or str as it has finesse property so would depend on their modifier. But now that I think of it I do t think off hand gets any bonus for ability modifier unless you have the appropriate fighting style from fighter etc.  

and is the longsword magic +1?

You've got a couple of things wrong:

• Two-Weapon Fighting (the action to use an off-hand weapon) requires BOTH weapons to be light, so no longsword.
• Off-hand attacks do not get the stat modifier
• (Remember that Shadow Blade is concentration, and as a caster with no shield in melee range he's more likely than most to lose it from damage)

But yes, Hexlock is crazy good at dealing damage compared to the other warlock subclasses.

Quote from doctormackerel >>

You've got a couple of things wrong:

• Two-Weapon Fighting (the action to use an off-hand weapon) requires BOTH weapons to be light, so no longsword.
• Off-hand attacks do not get the stat modifier
• (Remember that Shadow Blade is concentration, and as a caster with no shield in melee range he's more likely than most to lose it from damage)

But yes, Hexlock is crazy good at dealing damage compared to the other warlock subclasses.

Completely forgot that longswords aren't light and can't be used for dual-wielding (without the feat), good addition.

On a hexblade, shadow blade isn't a good pick because it's to-hit scales with strength or dexterity. You don't add an ability modifier to it, as the spell only states it does 2d8 psychic damage.

I'm going to assume your hex warrior is a human with Dual Wielder and the Improved Pact Weapon invocation for the d8+1 pact weapon. You'd be looking at about 21.5 (4.5+3.5(2)+4+2(2)) DPR...but by the time you've set this up the combat encounter is likely already over. On turn 1, you'd likely only deal 10.5 (4.5+2+4) if leading with the curse; 8.5-9 (4.5+4 or 9) if leading with SB.

The player would likely be better off going with a glaive/halberd + Polearm Master for a bonus attack that fully scales with charisma. I also believe hex outpaces hexblade's curse in damage until level 9. With this setup you'd get an expected 23 (5.5+2.5+7+4(2)) DPR by round 2. On top of this, you'd still deal about 13 (5.5+3.5+4) damage on turn 1. Note that this calculation does NOT factor in the reaction attack gained from PAM.

The key things are that shadow blade never counts as a pact weapon or as a hex warrior weapon so you can never use charisma with it. This means the hexblade has to use strength or dexterity to attack with a shadow blade and not charisma. Two weapon fighting does not add their stat value to damage unless you have the two weapon fighting style (which a warlock typically does not). The hexblade might typically be better off casting hex and then making two attacks with shortswords ... d6+1 + d6 +3 (assuming improved pact weapon for a +1 shortsword following by a two weapon fighting bonus action attack on subsequent turns for 2d6 damage. Both of these can use charisma since one can be a pact weapon and the other a hex warrior weapon.

Quote from Rexir2 >>

On a hexblade, shadow blade isn't a good pick because it's to-hit scales with strength or dexterity. You don't add an ability modifier to it, as the spell only states it does 2d8 psychic damage.

I'm going to assume your hex warrior is a human with Dual Wielder and the Improved Pact Weapon invocation for the d8+1 pact weapon. You'd be looking at about 21.5 (4.5+3.5(2)+4+2(2)) DPR...but by the time you've set this up the combat encounter is likely already over. On turn 1, you'd likely only deal 10.5 (4.5+2+4) if leading with the curse; 8.5-9 (4.5+4 or 9) if leading with SB.

The player would likely be better off going with a glaive/halberd + Polearm Master for a bonus attack that fully scales with charisma. I also believe hex outpaces hexblade's curse in damage until level 9. With this setup you'd get an expected 23 (5.5+2.5+7+4(2)) DPR by round 2. On top of this, you'd still deal about 13 (5.5+3.5+4) damage on turn 1. Note that this calculation does NOT factor in the reaction attack gained from PAM.

 

 The spell doesn't do damage, it creates a weapon that has a damage output. Just like all other melee weapons, you can still add your relevant ability modifier to the damage.

Jeremy Crawford agrees.

And unlike something like Flame Blade, it does not specify a melee spell attack that does x damage.

You're correct in that the spell creates a weapon, but the spell does state exactly how much damage the weapon does:

"You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). In addition, when you use the sword to attack a target that is in dim light or darkness, you make the attack roll with advantage."

2d8 psychic damage. That's it, no modifier. RAW, spells do exactly what they say they do. To compare, let's look at the minotaur's Horns trait:

"Your horns are natural melee weapons, which you can use to make unarmed strikes. If you hit with them, you deal piercing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike."

1d6 + your Strength modifier. This is explicitly stated.

EDIT: One more example with how on-hit effects work. Let's look at alchemist's fire:

"This sticky, adhesive fluid ignites when exposed to air. As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged attack against a creature or object, treating the alchemist's fire as an improvised weapon. On a hit, the target takes 1d4 fire damage at the start of each of its turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames."

On hit, it does 1d4 fire damage at the start of the target's turns. No initial damage, no modifier. It overwrites the general rule of 1d4 + Dexterity for improvised ranged weapons as per the specific-beats-general rule.

Don't get me wrong, SB is great! I use it on my tiefling dexterity fiend bladelock as it outpaces hex when used to TWF. As a dex build, initiative, dex saves and AC don't suck, either! Generally, though, hexblades won't want to invest heavily in dexterity due to their medium armor and heavy charisma focus.

Then again, if you, as the DM, rule that SB adds the attacking stat modifier to damage, that's your call. Unless you're running an AL game, go with what sounds most fun!

You're incorrect. Every weapon attack adds the relevant ability modifier to its damage rolls unless something specifically states otherwise (e.g. the bonus action attack granted by two-weapon fighting). You'll notice that the longsword entry in the Equipment chapter says it does 1d8 slashing damage. That's it, no modifier. This is because the rule that says you add your ability modifier to the damage roll is under the rules for weapon attacks, not the rules for any specific weapon.

He could also mouse over the Finesse link he bothered to post and read the tooltip.

Quote from Wtfdndad >>

He could also mouse over the Finesse link he bothered to post and read the tooltip.

 That would do it! I feel kind of dumb. Thanks for clearing that up!

This boosts the DPR for SB if it's the primary weapon by 2 or 3, edging out the d8 pact weapon until level 5's Thirsting Blade becomes available. Turn 1 would possibly to about 11-12 damage, 13-14 with Hexblade's Curse on turn 2, and 9 when used as the TWF bonus attack on turn 3. You'd be looking at 23.5-24.5 DPR on turn 3 and onward, assuming concentration holds up.

You can use hex warrior with Shadow Blade. The feature requires that you touch the weapon after you finish a long rest. This would require the warlock to cast it and to use it, which expends a spell slot. This benefit would last until the next long rest, at which point it would have to be repeated to keep the effect.

a little “game-y” but hay, we’re playing a game right? If a DM has a problem with it then they probably just won’t let you rest to get the slot back that quickly.

Unfortunately you'd have to cast Shadow Blade after you finish the long rest first, meaning hex warrior would not work. Even if it did, you would only have the hex warrior effect on it for the Shadow Blade's duration of 1 minute.

Thanks for the help! The +1 weapon was designed as a temporary boon, which worked out as intended for a great series of combat - though the overwhelming damage caught me off guard. 

The responses are greatly appreciated - i think i have a grasp on it now. Hexblade is more OP than I thought but the mistake was on me for 

1. Allowing a longsword (doesn't have light property), should be a 1d6 shortsword

2. Not following concentration rules

3. It looks like they CAN use their charisma bonus to both the shadow blade and the hexblade. 

Thanks!

They cannot get their charisma bonus on the shadow blade. Other than that, it seems like you’ve got a good handle on things.

Quote from Warranto >>

Unfortunately you'd have to cast Shadow Blade after you finish the long rest first, meaning hex warrior would not work. Even if it did, you would only have the hex warrior effect on it for the Shadow Blade's duration of 1 minute.

Okay okay hold on.... I mean, yes, obviously, the magical weapon created by Shadow Blade is only intended to exist whenever you've spent the spell slot to cast it. But uhhh... Pact of the Blade might be written in a way that actually gets around that???

Pact of the Blade

 

You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it (see the Weapons section for weapon options). You are proficient with it while you wield it. This weapon counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Your pact weapon disappears if it is more than 5 feet away from you for 1 minute or more. It also disappears if you use this feature again, if you dismiss the weapon (no action required), or if you die.

You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon. You perform the ritual over the course of 1 hour, which can be done during a short rest. You can then dismiss the weapon, shunting it into an extradimensional space, and it appears whenever you create your pact weapon thereafter. You can’t affect an artifact or a sentient weapon in this way. The weapon ceases being your pact weapon if you die, if you perform the 1-hour ritual on a different weapon, or if you use a 1-hour ritual to break your bond to it. The weapon appears at your feet if it is in the extradimensional space when the bond breaks.

So the bad news is... Shadow Blade only has a duration of 1 minute. Even with Extended Spell metamagic worked in, there just isn't enough time in the spell to allow you to complete the 1-hour ritual while holding the blade. But for the sake of thought experiment, put that out of mind for a moment, let's say we're dealing with a different magic item-summoning spell with a longer duration (case in point, Flame Blade is 10 minutes, which still isn't enough even when doubled... but who knows, maybe a similar 1-hour magic item spell or ability will come out at some point in the future?). The Pact of the Blade feature doesn't say you have to be holding a "permanent" magic weapon. After all your "pact weapon" is a mutable construct created by the power of your class feature, which is shaped by but independent of the actual item imprinted upon (which is why you can reform an imprinted magic sword to be a magic axe, something that's been debated in the past but is an assumption I'm comfortable making for the sake of this post). 

If there were some way to hold a Shadow Blade (or a similar spell) for one hour, you'd have...

1. Magic Weapon? check.
2. Held for one hour during a short rest? check. 
3. Not an artifact or sentient? check.

Which would allow you to "use your action to create a pact weapon in your empty hand," in the same form as it was when it was created by the Shadow Blade-like spell.

Clever, but nothing about how Pact of the Blade is written would prevent the shadow blade from ceasing to exist when the duration of the spell expires, even if you were somehow able to extend that duration to the hour that’s required by Pact of the Blade.

It could be a fun idea, but it’s definitely homebrew territory and more powerful than I’d be comfortable allowing to any of my players.

Sure, for the sake of argument let's say that technically you could use Pact of the Blade to do this on conjured weapons. However, even if it could stick around for the full hour to be converted, it would still only be a pact blade for the duration of the spell. Let's say you could quicken the pact to be instantaneous - Shadow Blade would still cease to exist at the end of 1 minute, ending the pact with it as there is no longer an object to summon.

I'm reasonably certain that Pact of the Blade will enable you to continue to summon a Pact Blade that has the same shape and qualities as the magic weapon you imprinted, even if that magic weapon is subsequently lost, dissipated, or destroyed. Destruction or loss of the item is not enumerated as a condition in "The weapon ceases being your pact weapon if you die, if you perform the 1-hour ritual on a different weapon, or if you use a 1-hour ritual to break your bond to it. The weapon appears at your feet if it is in the extra-dimensional space when the bond breaks." Or if not, completing the Pact Weapon ritual on the magic weapon created by a Shadow Blade or Flame Blade-type spell or ability would in my mind be a subsequent specific modifier that changes how that spell or ability normally operates. Like.... Summon Greater Demon or Conjure Elemental or Conjure Animals, these spells all have durations that are specified to be one hour, after which the creature "disappears". But when acted upon by Planar Binding, the 1-hour duration of such spells is no longer an issue, because the more-specific duration of Planar Binding takes over. The more specific duration of Pact of the Blade (the weapon can be dismissed and summoned forever, until the Pact is broken) in my mind supersedes the duration of the spell, even if one doesn't consider Pact of the Blade to be creating a wholly separate facsimile of the original blade. 

Certainly out on a limb, like I said I don't even think there's currently a spell or a feature published that this would work on, and if there comes to be one, the language will have to be more closely scrutinized then. But I think there's a thread there that is something to keep in mind as modules and source books are released.