Which spells to pick for bladelock at 3rd level?

[Mandragola]

There seem to be an awful lot of good level 2 warlock spells and I’m having trouble choosing mine.

My character is a variant human, going for a strength-based pact of the blade build. Stats are 17, 8, 14, 10, 8, 16. I took moderately-armoured as my level 1 feat, which is how I got to 17 strength. I’ll take heavy armour proficiency at 4th, to get up to 18. I’ll worry about whether to cap strength, raise cha or take PAM at 8th if I live that long!

So far I’ve mostly just been zapping things with EB, though I did have the opportunity to spend a few exciting turns on the front rank, defending a bridge against a horde of orcs at 2nd level. Hit one pretty hard with my mace at one point, though it proceeded to hit me back with its greataxe shortly afterwards. Having survived that, and killed a few orcs in the process, I find myself at 3rd level.

Spells I have so far are mage hand, eldritch blast, command, hex and armour of agathys. I’ve just hit 3rd level and have an opportunity to change things around a bit. I can drop a 1st level spell to get two second level ones.

So my questions are, which 1st level spell should I drop, and which two 2nd level spells should I learn?

I’m in a party with a dwarf barbarian, human monk, Halfling rogue (going for arcane trickster), wood elf life cleric and drow elf moon druid.

I will definitely keep hex, so it’s a question of dropping armour of agathys or command. So far I’ve never cast either of them, so it’s a bit hard to judge! Both actually scale very well, though command is probably not the best crowd control spell I could know. I can see a case for just keeping hex at 1st level eventually.

In terms of what spells to learn, there are so many options! Mirror image, scorching ray, darkness (nice potential combo with the druid in forms with blindsight – though as a drow she can do it herself), hold person, invisibility (which nobody else can cast), misty step (great escape for a melee character), shatter and suggestion are all contenders.

Thanks for your suggestions.

[Joe the Rat]

Which invocations did you grab?

Right now, unless your rogue is a big shooter, you're the main ranged fighter. Do you want to stick to that (Warlock with a side of stabbing), or try to be Gish Melee?

A major wrinkle in your picks is that anything with a concentration requirement will compete with Hex. Eventually a casting of Hex will last all day if you can keep concentration up, which may discourage using the other con spells. But if you aren't religious about Hex, you've got some leeway.

Invisibility is always useful. You can cast it one someone else, and it will scale up to four targets. But it does compete for concentration with Hex. But when you need to not be seen...

I'm guessing that when you're in melee, you're getting hit a lot. The two approaches here are mitigation, and avoidance. AoA is a good scaling mitigator - you can pop open 10 thp, and until that's gone, everything that hits you in melee takes 10 cold. Having that would encourage you to get up front. Mirror Image will help you at any range, but won't last long due to the low dex. You should use one, but it's kind of a split on which.

Hold Person would be a decent replacement for Command - you can stop/drop/flee/approach two people for a round, or paralyze one for up to a minute. The downside, again, is concentration.

Misty Step has a lot of use for you - as a Bonus action spell, it can get you out of, or in to melee and still leave the opportunity to whack someone.

If you need more ranged (and looking at your build, I'm seriously considering trying a Heavy Armor Tomelock Tank sometime), I'd look at Shatter as a temporary AoE option. Yes, it's a titch puny, and Scorching Ray has more versatility, but when you need to blanket an area with pain, this does well for being 3rd level. Added benefits include hurting things you cannot see, and the Con-based save means cover is not a factor. You'll still want to switch this to Fireball (for damage), Stinking Cloud (for control - watch concentration), or Hunger of Hadar (for a bit of both) at 5th.

My recommendation would be Invisibility and replace AoA with Mirror Image (defense) or Shatter (area damage). Keep Command for now, but have an eye to switch it later.

[Ruebin Rybnik]

Ouch two feats just to get Heavy Armor, that DEX is really hurting you.

Who is you pact with, what invocations do you have, and what sources are you allowed to use?

[Mandragola]

Thanks for the replies so far, especially Joe.

Fiend pact (so I already get temporary hp from time to time). I've taken devil's sight and agonising blast so far. Will of course take the bladelock invocations at 5th and 12th, if I make it that far.

At the moment I've been going around with scale mail and a shield equipped, so my AC is 15. That's kind of ok for a spellcaster, which is the role I've been playing. In future I plan to mix things up between ranged and melee combat, as appropriate, rather than specialising hard in either role. I expect I'll keep my shield for when I'm fighting things that are far away, as an AC20 "archer" (in plate with shield) sounds quite good.

I think of getting moderately-armoured and heavily-armoured as more like one feat than two, because they come with a built in +2 strength. My strength of 18 at 4th level gives me +4 to hit and damage, and no other build (with point buy) could do better. So it's actually not as huge a cost as all that.

Concentration is a really good point when looking at spells to take, which I've not thought about enough. It makes command (no concentration, great scaling) look quite a bit better. Likewise AoA and mirror image.

Concentration is also a good argument for grabbing polearm mastery at 8th. I could just have the third attack in melee rather than casting hex all the time. That would allow me to concentrate on something else. That's all quite a way off though for now, so I'll avoid concentration spells for now.

Invisibility is a really tricky one. It's so useful, but seems weird for a warlock somehow. Feels a bit like I'm trying to have it all. I do like the idea of shatter though - to of course be dropped in favour of fireball at a later date.

[Rebonack]

Oof, two feats for heavy armor. Was multiclassing banned in your game? Fighter at level one and then Warlock the rest of the way would have given you heavy armor from the gate as well as a Fighting Style to take advantage of.

Probably the biggest draw for Bladelock is the ability to pick up Great Weapon Fighting and pair it with Darkness and Devil's Sight to give yourself constant advantage on your attacks so you can hack away with that nasty -5/+10 bonus. As an added bonus, it makes hitting you with attacks nearly impossible and hitting you with any kind of targeted spell actually impossible. Given your build, the GWF shenanigans are a wash, but the Darkness/Devil's Sight nonsense would still be totally viable.

Remember that baddies don't get attacks of opportunity if they can't see you, so when using Darkness try to position yourself to split the enemy party. If their back-line can't see your friends, they can't shoot at them. Otherwise, make sure you have one or two foes outside the Darkness so your party can gang-up on them. Also keep in mind that two people INSIDE the darkness attack normally (advantage and disadvantage cancel out) so you can shout instructions to your pals to line up their shots.

[rbstr]

Seems like you're planning on being up front, given your plan for Heavy Armor and strength score.
If you're going to do that you'll probably want to bump up your defenses:
Mirror Image, even with a low Dex score, is still three hits that will miss you. No concentration. That's good!
It has anti-synergy with Armor of Agathys. So take one or the other, probably.
AoA will always work as THP, Mirror image won't work on things immune to illusions ect. AoA scales, but really Mirror Image doesn't need to scale.

Shatter is probably a good idea for the AOE blasting. Replace with Fireball when you can.
I wouldn't pick up Scorching Ray, Agonizing Blast is basically as good.
Darkness has really good synergy with your Devil's sight.

I'd seriously think about dropping Hex if you intend to be in harms way a lot though - it really isn't as good as the white-room would put it on a character that'll take hits.
Think about it this way: You could cast Scorching Ray for 6d6 (21 average). If you hexed a target and smacked it with a weapon that's 1d6 + 2d6 (greatsword)+3 (13.5).
So you need make 3 more Hex-buffed hits to exceed Scorching Ray's damage for that one slot. How long are you really going to be able to keep it up if you're being targeted?
Against fireball, where they still take half-damage on a save, it's a really tough sell on Hex.

[Citan]

Misty Step has a lot of use for you - as a Bonus action spell, it can get you out of, or in to melee and still leave the opportunity to whack someone.

My recommendation would be Invisibility and replace AoA with Mirror Image (defense) or Shatter (area damage). Keep Command for now, but have an eye to switch it later.
I concur with Invisibility, although Suggestion is aso a great one to have.

Mirror Image though? A VERY BAD IDEA for OP, who has a FRIGGING 8 DEXTERITY.
So, besides the fact you still get the brunt of attack around 30% of the times when spell is first cast, with AC 10-1=9, the duplicates would last one round at the best of the best. And probably no more than 1 and half turn at worst (with enemies getting Multiattack soon).
Plus this spell does not scale.

OP, if you want really good defense, better grab Devil's Sight Invocation and cast a Darkness onto a stone or something near you: you can see (most) others, (most) others can't see you. You cannot have Hex at the same time, which is sad, but you won't make any better defense than that for quite some time. Plus it can be used in many other ways (create a distraction, provide a cover when sneaking, throwing the stone to set confusion in a group of enemies, etc).

[Talionis]

You can drop Armor of Agathys and pick it up later when you can up cast it as a 3 or 4th level spell. It works great if you can get a Cleric to cast Warding Bond on you so that you have Resistance to Everything. AoA is awesome, but it takes some combos to keep it up long enough to make it worth the action economy to cast it. But until you can cast it at a higher level and it does meaning ful damage and you have a mechanic to help it stay around long enough to fire off multiple times, AoA isn't worth it. AoA also will have some non-synergy with your Fiendish Temporary Hitpoints since Temporary Hit Points don't stack.

I think Shatter or AoE damage is a good pickup its best to soften up your targets.

If you want to wade into the front lines then I'd suggest a dip in Cleric or Fighter to get Heavy Armor. Heavy Armor Mastery is a surprisingly worth it feat on a dedicated Armor of Agathys user.

[rbstr]

Mirror Image though? A VERY BAD IDEA for OP, who has a FRIGGING 8 DEXTERITY.
So, besides the fact you still get the brunt of attack around 30% of the times when spell is first cast, with AC 10-1=9, the duplicates would last one round at the best of the best. And probably no more than 1 and half turn at worst (with enemies getting Multiattack soon).
Plus this spell does not scale.

That's not a really good analysis of the spell. If you're fighting something that's more likely than not to hit you you're gonna come out ahead and not "waste" many images. So it's best against the meanest critters out there. It will usually eat more damage than AoA can in that situation.
AoA on the other hand is best against things that don't hit as hard or will get hit less often since it doesn't get wasted.

No doubt Mirror Image is better if you have high dex. But it's far from a bad idea.

[Citan]

That's not a really good analysis of the spell. If you're fighting something that's more likely than not to hit you you're gonna come out ahead and not "waste" many images. So it's best against the meanest critters out there. It will usually eat more damage than AoA can in that situation.
AoA on the other hand is best against things that don't hit as hard or will get hit less often since it doesn't get wasted.

No doubt Mirror Image is better if you have high dex. But it's far from a bad idea.
Sorry but you are dead wrong here.
AC is 9 with DEX 8.
Even the lowest critter still has at least +3 to hit.
Most critters level 3 players would encounter will have +4 or +5.
Any ranged attacker of that level has at least +4.
Any serious "weapon attacker" of that level will have a +6 or +7.

Several CR1 enemies (IIRC) have Multi Attack. It's a given past CR 4/5.

So best case, enemies have "only" (lol) 75% chance to hit a duplicate if you rolled the redirect. Worst case, they have ~90% chance to hit a duplicate.

So, for now OP having AC 15...
Chance to avoid hit "normally" hit in best case (only +3 to hit): 55% chance.
Chance to avoid hit thanks to Mirror Image: 75% first time (3 duplicates), 60% second time, 45% third time.

Now when OP gets level 4 and get 18 AC... Against same enemies, chance to avoid hit will now be 70%: so very close to the best case of Mirror Image (3 duplicates), and plain superior as soon as 1 duplicate has been removed. Stack a shield and it's now 80%, so Mirror Image is plain worse than doing nothing.

Let's say you are lucky and always roll enough to get duplicates attacked instead every time in chain.
If you faced only one powerful enemy and wanted to avoid him, it's worse than just using Misty Step to fall back then use Eldricht Blast.
If you faced two enemies, both will get OA if you move, probably immediately removing one of the duplicates before their turn even started.
If you face ranged enemies, they will laugh away while crippling your duplicates.

Seriously, that's a big waste of a spell slot, which comes scarcingly for a Warlock. If you really want defense but keep Hex...
- Learn Blade Ward worst case to use only in dire situations.
- Learn False Life invocation and spam it every encounter start.
- Learn Repelling Blast and try your luck (disadvantage) before moving away.
- Use Minor Illusion to create a cover or maybe try and make yourself so frightening you bluff the enemy into changing target.
Those are not great tactics because they eat your action that would otherwise spent on dealing damage, but good enough for those few cases when you are in danger.
Also...
- Use Hex to target STR or DEX depending on creature and shove it prone to get advantage on your attacks and impose disadvantage on OA. Requires to know the creature beforehand though.
- Cast a Blindness on the creature (Fiend): great for the whole party, and non-concentration.

In practice, if a Warlock REALLY wants good magic-based defense with low DEX, but does not want to use Darkness for any reason, either stand with Armor of Agathys or take a dip into Sorcerer for emergency Shield and Shocking Grasp (multiclass for Blur)... Or use Phantasmal Force if only a creature is concerned.
Or just Misty Step back behind a Mold Earth/Minor Illusion cover you crafted at the start of the encounter then use your action to Hide.

Sorry, but Mirror Image is real crap for a dumped Dex Warlock, especially a STR Warlock which has good AC already thanks to heavy armor, because it won't make any difference against small or bigger crowds, will improve only ever so slightly your chances of avoiding attack from an enemy, while still using 50% of your potential greatness until next short rest.

[JakOfAllTirades]

Having used Mirror Image quite a few times, I have to disagree with the analysis above. It doesn't matter what your character's AC is. And it doesn't matter how long the spell lasts. This is simple: You get three duplicates, and they're all good for exactly one hit. Each hit taken by a duplicate is a hit not taken by your character. Against a powerful foe, that can add up to a lot of avoided damage. Is Armor of Agathys better? Maybe... but at 3rd level it only stops 15hp. Mirror Image will likely prevent more than that, sometimes in one hit.

I'd take it.

Edit: Misty Step is your friend. And Suggestion is all kinds of fun.

[Mandragola]

Sorry but you are dead wrong here.
AC is 9 with DEX 8.
Even the lowest critter still has at least +3 to hit.
Most critters level 3 players would encounter will have +4 or +5.
Any ranged attacker of that level has at least +4.
Any serious "weapon attacker" of that level will have a +6 or +7.

Several CR1 enemies (IIRC) have Multi Attack. It's a given past CR 4/5.

So best case, enemies have "only" (lol) 75% chance to hit a duplicate if you rolled the redirect. Worst case, they have ~90% chance to hit a duplicate.

So, for now OP having AC 15...
Chance to avoid hit "normally" hit in best case (only +3 to hit): 55% chance.
Chance to avoid hit thanks to Mirror Image: 75% first time (3 duplicates), 60% second time, 45% third time.

Now when OP gets level 4 and get 18 AC... Against same enemies, chance to avoid hit will now be 70%: so very close to the best case of Mirror Image (3 duplicates), and plain superior as soon as 1 duplicate has been removed. Stack a shield and it's now 80%, so Mirror Image is plain worse than doing nothing..

This analysis is based on misunderstanding the mechanics of mirror image.

You roll to see whether the attack targets the spellcaster or the image. Then you roll to hit as normal, against the AC of the target. So if I'm going around with AC 18 or 20, and an attack targets me rather than an image, it still has to hit my AC of 18 or 20. It only targets the image's AC of 9 if it targets an image.

To be honest, I actually doubt whether I'll be casting either AoA or mirror image much, at least at the moment. There's a case for AoA right now, as I'm fighting goblins and they will die outright if they hit me in melee. But at the same time, the 6 temporary hp I get whenever I kill something is kind of sufficient against goblins, and gets refreshed very often. But basically I have only two spell slots and neither of these spells seems a great use of them right now.

Nobody much in the party seems to have AoE spells, so shatter makes sense. It will be swapped out for fireball later. I think our arcane trickster will eventually pick up invisibility, so I think I'll leave that out for now. Scorching ray probably isn't worth it when I have EB+AB (though it does scale really well...). So that's a few options eliminated.

I think in the end I'll just keep hex as a first level spell in the end. I'll avoid other stuff that requires concentration, unless it's an encounter-changer like fear.

[Citan]

This analysis is based on misunderstanding the mechanics of mirror image.

You roll to see whether the attack targets the spellcaster or the image. Then you roll to hit as normal, against the AC of the target. So if I'm going around with AC 18 or 20, and an attack targets me rather than an image, it still has to hit my AC of 18 or 20. It only targets the image's AC of 9 if it targets an image.

No, I PERFECTLY UNDERSTOOD how it works. It's exactly how I described it: once you get AC 18, the chance to avoid a hit against crowds thanks to a Mirror Image is just a tad better with 3 duplicates, and plain worse as soon as one is erased. And the probabilities took that into account. Against foes that would have a very high chance to hit you, yeah, that will indeed save you from up to 3 hits. IF those foes are the one attacking you.

@JackOfAllTirades: Mirror Image can be worth it even with dumped DEX when you are a "normal" fullcaster, which has many slots and ability to recover some, so you can use it as an alternative to Shield for a bit longer protection, in the niche situation where there is only one powerful enemy threatening you during several turns.

In every other situations, it's as good as creating a Minor Illusion (against archers) or Dodging/Disengaging (against melee).
Because, again, a handful of attacks (not hits, ATTACKS) will be enough to disperse duplicates, due to the very low AC.
You are considering the best case, which is 1. You systematically roll well enough to get duplicate attacked instead and 2. It's a very powerful creature that attacks, so it's a heavy blow that you would have taken instead.
You have very little control on this though, because you have little way usually of influencing enemy behaviour.
You may have archers raining arrows beforehand, or just smaller creatures that accompany the big powerful try and encircle you beforehand.

If you are tanking, it's worthless with a dumped DEX, comparatively to ANY other option a Warlock could use his 2nd slot with. Blur uses concentration but imposes disadvantage, same with Darkness, and both last the same time.
Let's put an example: creature with +9 to hit, against AC 18.
If it attacks the duplicate, it's an autohit.
It normally has 60% chance to hit you.
With Mirror Image active, you have 75% to divert the first hit, then 60% chance to divert the second, then 45% chance to avoid the third.
Such an enemy would obviously have Multiattack, so basically, in the best case, Mirror Image would make him auto-miss a full round (Multi Attack + Opportunity Attack). Now what? You diverted a full round of attacks, provided it was a duel. And now you are back with someone having 60% chance to hit you. If you stay in its reach you will probably suffer, unless it can be killed this round.
Compare with...
- Shield (Sorcerer dip): 2*long rest, +5 AC, putting his chance as low as 35%, BUT ALSO WORKS against any other attack (so no loss efficiency against crowds/archers/casters).
- Blur: disadvantage: put this enemy's chance as low as 36%, works against any attack, lasts for up to a full minute.
- Darkness: same effect as Blur IF enemies can even see you. Same with archers, same with casters. At best, you won't even get attacked. Of course, if you want to "tank" (aka aggro attacks), it's obviously not a good idea. XD

My point is this: either you really want to stand your ground on the front line, then you get a really useful option to spend half your resources on (remember, only 2 slots until 11th level). Or you stay a Warlock (like a Rogue, avoid staying in melee) then you really don't need it because you get Misty Step to avoid getting overrun.
Because theorycraft situations in which you are dueling one powerful opponents are very rare occurences, unless you and whole party is investing heavily into creating its. And being extra good at surviving which amounts to at most 1-2 turns worth of attack is not "tanking".

[Mandragola]

Ok, so you understand mirror image - but not maths. It's never worse to have mirror image up and it's wrong to suggest that it could be. No matter how bad an image's AC is, it's still a chance that an attack fails to target you in the first place. That attack that may have hit or even critted you - or it may roll a 1.

Mirror image gives you a 75% chance to avoid a hit when you have 3 images, because the image gets hit instead. AC gives you a chance to avoid a hit as well. Let's call it a 75% chance again, for the sake of simplicity.

The two chances are cumulative. 75% of attacks go after images. 25% remain to go after the PC. Of those 25%, the attacker has to roll to hit as normal, and 75% are avoided by AC. 25% of them actually go through to hit the PC.

This means that the final chance to hit a PC with 3 images up and an AC that avoids 75% of attacks is 25% of 25% - or 6.25%. That chance increases as you lose images.

The mirror images of a low-dex PC are easy to hit. But that doesn't mean that monsters will always roll low against them. Sometimes they'll even crit them. It's true that a lot of attacks that destroy images wouldn't have hit a high-AC PC in the first place, but some of them would.

There are a lot of problems with mirror image, as you point out. There's no saying whether the images stop a goblin's arrow or a giant's club, so it can often be wasted (and can occasionally be awesome, say in a room containing just one giant!). I've already decided it's not a good use of one of a warlock's two spells. But let's at least make the decision based on facts.

Overall I think this makes mirror image a really good spell for a wizard or sorceror, who should only get attacked occasionally, has a relatively low AC and has a lot of lvl 2 slots to play with. I don't think it's a good spell for a warlock with a high AC.

[rbstr]

Exactly - it massively reduces your chance of getting hit regardless of anything else.

Sure there are "false positives" where what would have been a miss takes out an image. But that means it's just like any other spell - sometimes it's not a best thing to do.

As a font-line fighter with low dex it's particularly bad against the riffraff that'll miss you often and don't hurt very much. But against something that is likely to hit and to hit hard, like a Giant, it's pretty good.

[BW022]

I've run a couple of bladelocks (one strength-based). My advice is:

Drop command and take darkness and spider climb.

Darkness is a mixed bag. It typically nerfs your party and slows combat to a crawl so you probably will never use it... but... it is useful as a 'get out of jail' spell. It can virtually reset the entire combat -- given the party time to pull back, heal, escape, avoid melee, or avoid ranged engagements. It has some uses like invisibility -- you can sneak in the dark fairly easily. It is also a reasonable distance crowd control spell... not as good as others but it has flexibility. Combined with your devil's sight, it is pretty much solid advantage for you and disadvantage for enemies... assuming you are far enough from the rest of the party.

Spider climb. You have a serious AC problem which you can't really fix. Even plate (AC 18) isn't totally great for melee at 4th. You typically don't want a shield (since it prevents casting of spells with a weapon in your hand, baring paying the warcaster feat tax). So... my advice is to summon a glaive use spider climb to go up a wall or ceiling and attack with reach. Most creatures (at your level) won't have reach or ranged weapons and this often allows you to attack with immunity. It obviously has utility ability as you can scale cliffs and such with it and it can be cast on other party members. At 5th-level, replace this with fly.

I would keep armor of agathys. And I would cast it anytime you expect trouble. It is worth the slot. It doesn't have concentration and lasts and hour. It is effectively 10hp to you and often 10hp or more damage to your opponents.

[Mandragola]

Thanks everyone for the advice. I've given it a lot of thought. Options have been narrowed down:

Shatter seems a good option. Nobody else in the party has an AoE spell that I'm aware of. It'll be replaced by fireball in 2 levels time but for now is handy.

Given my role as the only arcane caster in the party, I don't think I'll ever cast buff spells. I simply need the slots for hex, fireball and counterspell - which are the spells I'll mostly cast I think. This effectively rules out AoA, mirror image and invisibility. Effectively I'm making some decisions that I might not usually make in favour of the party's overall strength.

There's a conflict between hex and other cc abilities, which tend to require concentration. As a result I'm considering taking PAM at 8th level (quite a way off still, obviously). I think that by that time I'll often want to be using my concentration slot on something other than hex, so my bonus action will tend to be free to bash people.

So I think that my second choice will be misty step, to replace AoA.

[Citan]

Ok, so you understand mirror image - but not maths.
There are a lot of problems with mirror image, as you point out. There's no saying whether the images stop a goblin's arrow or a giant's club, so it can often be wasted (and can occasionally be awesome, say in a room containing just one giant!). I've already decided it's not a good use of one of a warlock's two spells. But let's at least make the decision based on facts.

Overall I think this makes mirror image a really good spell for a wizard or sorceror, who should only get attacked occasionally, has a relatively low AC and has a lot of lvl 2 slots to play with. I don't think it's a good spell for a warlock with a high AC.
SERIOUSLY, I PERFECTLY UNDERSTAND THE MATHS (which by the way I alone provided with factual case, contrarily to others)!

I demonstrated that having a chance to avoid up to 3 hits is absolutely not worth a 2nd slot when OP has already a pretty decent AC, and can grab other more durable defensive options. Because he is still not built as a tank, so he is not supposed to try and threaten the biggest guys anyways. Plus it's very unreliable since you don't know who is gonna attack you, so the duplicates could be "wasted" on small creatures that go along the big one, or some archers raining arrows on you.

Check back, I never told that "Mirror Image is a bad spell" or something like that.
I TOLD MYSELF that Mirror Image could be worth for a Wizard. NOT FOR A WARLOCK with 8 AC who has ONLY 2 SLOTS PER SHORT REST (especially since it's not even a scalable spell)!
Any other spell will be a better use of that slot. Really, ANY. I even detailed all of them.

[JakOfAllTirades]

@Citan - your analysis is correct, but you may have missed the caveat in my post; "against a powerful foe" is the situation for which I recommended Mirror Image. In a solo fight, or against a small group of high-CR monsters, it's likely to prevent a lot of damage. Against a horde of low-powered enemies, it's less useful. The question for a Warlock is at what point do they know enough spells to include both; Mirror Image and <other spell> for defense against hordes.

[Laserlight]

I've played a bladelock (Fighter 1/ Lock X) from L3 to L6 and have found Darkness plus Devil'sSight works well. Despite the warnings you always hear, Darkness has almost never interfered with the rest of the party's attacks; all you have to do it pay attention to your maneuvering. I get constant Advantage on my attacks, Disad on enemy attacks, immunity to spells that specify "any target you can see", I don't provoke Opportunity Attacks, and it intimidates friend and foe alike when a cloud of darkness envelops the hobgoblin captain and passes on, leaving a pile of hobgobbets behind. All that is worth much more than Hex to me.
(Although in a few levels when Hex gets to be 24 hours for one spell and Darkness is still only a few minutes duration, I may reconsider).

Also...starting with one level of fighter or two of paladin would give you heavy armor, a fighting style, and other benefits.

[UrielAwakened]

Why not just be a Hexblade lock? It would have saved you a feat and you'd already have heavy armor proficiency?

[Mandragola]

Why not just be a Hexblade lock? It would have saved you a feat and you'd already have heavy armor proficiency?

We don't use UA material. Hexblade does look quite cool, I admit, but as a rule our group doesn't like to use splat content. Personally when I DM I follow the adventurer's league system of core book +1 source, but the guy DMing this campaign is sticking just to the PHB.

He also dislikes multiclassing, as do I, and conversely I enjoy the challenge of making a viable bladelock that's a single class.

Actually I don't think I lose much by not multiclassing. Yes, I've taken two feats. But at 4th level I'll have one of my primary stats at +4 and the other at +3. That's actually about as good as any build can be any way (other than a mountain dwarf) even without feats. I've got 14 con, too. My tertiary stats have suffered, but who really cares? Wisdom is just another word for cowardice, in my book*.

The positive is that I get full spell progression as a warlock, with a second attack at 5th level (and fireball and counterspell), 5th level spells at 9th level and lifedrinker at 12th. I don't like to wait around for core character features just to get stuff I can grab elsewhere.

@Citan, fair enough. We agree mirror image isn't a good spell for a warlock. In practice, even against a single strong character, I'd be better off doing almost anything else on the first turn of a fight than cast mirror image - using up 50% of my spell slots to do so.

*Until someone casts fear at me, but hopefully by then I'll know how to cast counterspell!