So, making random characters is a thing I like to do sometimes. A while ago, back during the days of 4E, I had quite a few character concepts knocking around. This was true even though I never got to play in a single game, as I was DMing.
One of these was a fiendish warlock idea. The 4E warlock, for those who don't know, had its own array of class-specific powers like all the other classes, and this meant the different warlock pacts could really go to town with thematic abilities. All of them were interesting, but the fiend stuck with me. It had powers like summoning demonic claws to rend people, drain their life to heal yourself, curse them and wreck their mind with terrible illusions until they carve themselves up, feed (bits of) your own soul to a demon to harm an enemy... Good stuff.
The idea that particularly appealed to me was someone who completely accidentally ended up in a fiendish pact. It's just nicely different. Specifically, I had this idea for a servant in a rich household, who stumbles across a diabolical ritual and ends up receiving the fiend's gifts in place of her master - because of course, fiends love to twist the rules. Naturally, she's then left fleeing from a sinister devil cult whose members could be lurking anywhere. I liked the idea of someone left to defend herself reluctantly with unwanted evil powers, and probably ending up in a destructive spiral.
Since 5e features the warlock too, and equivalent pacts, I thought I'd try building her here. Our current campaign is about 5th level, so I decided to go for a 5th-level character. Also, this tends to be quite enlightening about your capabilities, whereas 1st level can be somewhat uninformative.
After a while bashing stats around, I realised that it's actually quite difficult to build this warlock. There are two problems here.
The first and primary problem is that essentially, the point of being a warlock is that you cast hex on people and maintain it more or less permanently to boost your damage, and then use eldritch blast to deal said damage. Eldritch blast allows you to hit multiple targets as you level up, rather than increasing damage for one target; the warlock also has a choice of beneficial upgrades that affect only eldritch blast, not any other cantrips. In particular, you can take an option that adds your Charisma bonus to each hit, which together with its long range quickly makes eldritch blast the deadliest cantrip out there - you can very quickly be dealing 1d10+3+1d6 (as good as a 1st-level spell), and this increases rapidly, hitting 2d10+2d6+6 at 5th level (average 24). Another option adds pushback, although there are fewer upgrades in 5e than the 3rd edition warlock. Warlocks are really good at this.
However, the flipside is that the warlock has very few other options, and most are flat-out worse from a mechanical perspective.
Hex can be maintained more or less forever, barring a failed concentration check. It transfers between targets on death, unlike any other spell except the similarly-intended hunter's mark, and it can lie fallow between combats only to be resumed as needed. Of the warlock's handful of other spells, nothing else comes close to being this all-round useful. Don't get me wrong, there are good spells in there, but the trade-off against hex's always-on damage boost is a heavy one. It seems pretty clear that the core warlock design, where spells are regained on a short rest but you have very few, is intended to ensure that warlocks can always have hex available, with other spells being a handy extra, a niche effect or a utility slot.
Added to this is the fact that hex takes your concentration slot, which means it's a pretty suboptimal decision to focus on any other spell that requires concentration. Charm person, hold person, fly, anything that enchants or boosts or does pretty much anything other than damage is unwise. Casting one of these will interrupt your hex, meaning you'll lose a long-term benefit for a short-term one that might not even work. After all, most of those require saving throws, and often allow multiple saves (as 5e has wisely tries to cut back on stunlocks), and only work on certain targets, whereas hex just flat-out works. Since you have very few spells known in the first place, choosing these is a big gamble or commits you to what's probably a suboptimal playstyle.
The scaling spellslots helps - casting hold person at 5th level is nice. But is the possibility of paralysing four humanoids (who get saving throws every round) worth the tradeoff of guaranteed extra damage against all creature types for basically every attack you make until your next short rest? Opportunity costs become a big concern for the warlock. Don't get me wrong, sometimes one of these spells will be exactly what you want. The difficulty is the combination of very limited spells known, the fact that casting any concentration spell means you lost a huge damage buff, and the way virtually any spell has a much more niche use than hex. There's a lot of reasons to favour simple, widely-applicable non-concentration spells.
The second factor is that the warlock's choice of spells is very limited, and specifically their cantrips. There's no solid alternative to taking eldritch blast. You can, of course, but you'll lose a lot of offensive capability without having much way of compensating. The fact that you can't use invocations to boost anything but eldritch blast particularly discourages any other approach. Your non-combat cantrips are minor buffs that don't offer much active capability, so there's not really any other obvious combination of abilities to build a playstyle around. If you aren't zapping things with eldritch blast, what exactly are you going to do? And if you are doing that, then taking hex is very much the optimal choice. And if you're using hex, then casting other concentration spells is nerfing yourself.
This is, I think, possibly a mistake? It seems to lock the warlock into a single niche far more firmly than any other class. The pacts offer a few more spell options, but don't fundamentally change the way warlocks work. Again, several are concentration spells that seem a poor choice given the fairly clear assumption that warlocks are running hex.
Dude, where's my curse?
So the other issue I ran into was that while I loved the flavour of a fiend-pacted warlock, the expected warlock mechanic of constantly hurling bolts of magical energy at people is very much not what I had in mind. Does that spell "sinister pact with a demon" to you? I mean, it can, there's plenty of basis for evil-powered superhumans, but that's not the image I get. I'm thinking darkness. I'm thinking lies. I'm thinking deception, and head games, and dread, and power over people, and blights and curses and afflictions. Sure, I'm also thinking fire and brimstone, but bolts of magic? Not really. Spooky young girl pactee does not hurl force lightning. She preys on your fears, or she conjures up claws of darkness to drag you into hell. Honestly, the bolt-hurling thing almost feels more like sorcery, all about inner reserves of raw magical power.
In all honesty, quite similar things apply to other pacts. If you've made a pact with things from beyond space and time, I expect you to warp reality and drive people insane, and conjure up monstrosities, rather than blast them with lasers. Fey, of course, are notorious for raining bolts of eldritch power down upon people - oh no, wait, they wrap people in illusions and transformations and enchantments, and turn the wild against them. The various pacts give you a slightly different group of spells to choose from, but don't seem to significantly change your capabilities.
In fairness, again, this is partly because warlocks depend on two different subclassing mechanics, the Patron and the Pact. Mostly the breakdown seems to be melee-based warlocks vs. zappy warlocks, with their patron-flavoured abilities mostly subsidiary.
It's possible to eventually burn an invocation slot to buy the ability to cast either bane (a level 1 spell) or bestow curse (a level 3 spell) once per day, using a spell slot in the process from your incredibly limited supply. That is a very expensive ability. It is, of course, getting auto-levelled to 5th level, which makes it quite good. Probably not good enough to be worth losing a spell slot and an invocation to gain a decent debuff on up to 4 enemies at the cost of (once again) dropping hex, though, to be honest. Bestow curse is a little better, but has similar issues - notably, one of its uses is to essentially duplicate hex on a single target.
Other invocation abilities include things like at-will illusion, seeing through even magical darkness, levitation, at-will armour. These don't burn any spell slots and can be used constantly. The bane spells are useful, no doubt about it, but it feels like an extremely begruding tradeoff that's strictly worse than these abilities.
Building a Servant of Darkness
I spent quite a while trying to knock my warlock into shape. I faffed about with the Arcane Initiate feat to obtain alternative cantrips. I played with multiclassing. And then I realised I was doing it wrong.
I want my warlock to whisper dark secrets that drive you mad, to bend people to her will, to hold them helpless, twist fate against them, blight them with afflictions. I don't want her walking around energetically hurling magic; I want a simple glance from her demon-lit eyes to send them fleeing.
You know who can do all that stuff? The bard.
I'm serious. Look at the bard spell lists. For a start, let's note that bards get far more spells because they're designed for a different niche. We begin with vicious mockery, rather weak (1d4) as cantrips go but with a reasonable rider of disadvantage for the target, and needing only verbal components. Very flavourful, just what I wanted. Minor illusion is a good extra here for those "Efficiunt Daemones, ut quae non sunt, sic tamen quasi sint, conspicienda hominibus exhibeant" moments. On the real spell front, we have bane, charm person, disguise self, dissonant whispers, faerie fire, feather fall, Tasha's hideous laughter, crown of madness, enthrall, heat metal (how daemonic is that? frying someone in their own armour?), hold person, suggestion, bestow curse, fear, speak with dead (being dead is no defence against a demon), animate objects, geas, eyebite...
Even most of the other bard abilities feel appropriate. Being surprisingly good at all skills? Drawing on diabolical knowledge. The bardic inspiration ability feels a little odd, until you get the College of Lore and use it exclusively to make your enemies fail at everything by mocking them, which fits perfectly. The only one that seems a little odd is the free healing for allies, and you can view even that as being just one of the many fringe benefits of association with diabolical power, call now to see how much you could gain, operators are standing by! Essentially it's the patron advertising to the character's social circle.
Annoyingly, there's still a few very evocative spells that aren't available to bards. Flesh to stone, create undead, any ability whatsoever to get an actual demon to help you. The fire end of things is very limited. Still, it seems a lot better than the warlock at portraying the classic servant of demonic powers. Ironic really.
My current inclination is actually to think that multiclassing is the way to get all the key spells, but that bard needs to be the basis. The Magic Initiate feat looks promising, as it would give access to produce flame (but none of the druid 1st-level abilities are very thematic) or to thaumaturgy and command from the cleric list, both of which are highly desirable. As usual, the best option for spell breadth is to multiclass into wizard and pick up those lovely fire spells. This would, amongst other things, help reduce your dependence on enchantments - I don't have the current monster manual, but in older editions a worryingly large number of things were immune to these spells. The light domain cleric is also potentially a very nice match, if you completely ignore all that fluff about deities.