In my session I have a Warlock with a Charisma score of 20 that's been using the Eldritch Blast spell with the Agonizing Blast eldritch invocation to rather spectacular results, decimating everything that I've thrown at the level 8 party.
At first I was pleased at the party's success in difficult encounters, however I've discovered over time that the consistent, high, and nigh-uncounterable force damage of this cantrip tends to trivialize most encounters, eliminating the need for creative thinking. Combine this with a School of Evocation Wizard and I've found that most encounters are swiftly concluded with an initial Fireball and a final barrage of Eldritch Blasts, with any significant damage being eliminated by their druid's healing.
Unfortunately I've found that my current approach to dealing with their combat effectiveness has led to increasingly volatile encounters. I've been forced to serve them encounters meant for significantly higher level parties, and I've found it difficult to find a middle ground where the party is challenged but not always at risk of being totally wiped.
What is a strategy that I, as a DM, can use to counter the effectiveness of the Agonizing Blast Warlock?
I should emphasize that I'm not interested in crushing my players. I simply am looking for a way to force the party out of their singular, overwhelmingly effective approach to encounters.
I'm not interested in making the Warlock useless either. This is a spotlight issue, as the Warlock's build often allows him to be the singular source of damage for the entire encounter. Unfortunately this tends to invalidate the contributions of the other players, so I'm mainly looking for a way to reduce his effectiveness so that everyone involved can feel valuable and engaged in a fight.
Bonus points if you know of a strategy that fits into a primarily undead/fiend-focused campaign.
Best Answer
There are a number of answers:
I speak as a current player of a 5e warlock, who has 20 cha and agonizing blast, and who does not at all feel like an overwhelmingly dominant force on the battlefield. I manage to do my part, but "trivialize" would not describe.
Melee troops getting up close and personal with the warlock. If you're standing next to an enemy, your ranged attacks (like eldritch blast) are at disadvantage. If you move out of reach, you take opportunity attacks, and then they move up to you on their turn again. Also, Warlocks are generally not so hot at AC - getting some damage pressure on them will make them unhappy campers pretty quickly. Finally, a number of the shinier warlock spells (Hex, for example) are concentration-based. If you're whaling on the warlock a few times a round, those will drop pretty quick (and the warlock themselves not all that long thereafter). There are a number of high-mobility melee monsters out there you can use for this. Open spaces are also useful for making sure that the party fighter/paladin/barb can't block the foe all that effectively.
Fewer, tougher encounters in the day. The thing the warlock does really well is sustained, all-day damage. They're not necessarily any better at it than other sustained-damage types like the rogue, ranger, or fighter, but it is the thing they're good at. EB with agonizing blast is good. It's Class Feature levels of good (because that's basically what it is). It's not game-breaking good, though. It caps out at 4 attacks of 1d10+5, with maybe another 1d6 per if you're juggling Hex effectively. If he's the superstar of your party, then it's probably because you're letting him do what he does best - chew up lots of little encounters where other casters have to be careful with their spells. Crank up the threat level of individual encounters, to the point where the party can only handle 2 or 3 in a day, and he'll start sweating. If you keep him from having a short rest in between, he starts sweating more. If the entire party is coasting, then maybe just up the difficulty level by a notch or two.
Use cover. If the enemy has cover to hide behind, then you either have to take the penalty to hit, or you have to get up close. You can be fair, and give the party cover too, but if the party needs to take out the foe with a time limit, and the foe isn't using ranged attacks, that won't matter but so much.
Allow struggling party members to rebuild their characters a bit, perhaps with advice. If the warlock has a 20 cha, then every other character who hasn't used a stat-up to get a feat should also have a 20 in their primary stat. It's possible that your issue is that this one character is reasonably well-optimized and the others just aren't. If that's the case, then you should be able to rebuild the others a bit so that they're more in line with the expected level of optimization... and then crank the difficulty across the board.
Worth noting that it can be taken too far. If the warlock gets to trivialize the occasional encounter (with the other party members obviously contributing), and everyone involved enjoys it, that's great! It can be even better if you can make those the "humorous interlude" encounters, and play them for entertainment value. Point is, if everyone's mostly enjoying them, don't lose that kind of encounter just because you're trying to put together encounters that are not trivialized.
Also worth noting that some of these answers have counters of their own. Spell Sniper feat lets you ignore partial cover. Crossbow Expert feat lets you make ranged attacks in melee without disadvantage. There are a number of ways to make concentration checks easier. In general, though, these take major build resources. While the warlock is shoring up these weaknesses, hopefully your other PCs are finding ways to improve themselves as well. Acknowledgements to @ShadowRanger for the point.
There is another side of it, too, in that you might want to look into tweaking the power levels and effectiveness of your other party members.
Spells are simply much less gear-dependent than weapon combat. If you're severely limiting the party access to magical items, particularly magical weapons, then any weapon-dependent party members you might have are likely to fall behind your primary spellcasters. Similarly, there are magic items that you can add in that will improve survivability without significantly improving damage dealt. A low-magic campaign is fundamentally grittier in some ways, and it's not surprising that the combat gets swingy.
Fireball itself is significantly more powerful than other spells of its level, and excels at taking large chunks of HP off up front. Eldritch Blast, as a reasonably powerful ranged multiattack, is quite good at picking off what remains. It's not as good at chewing through full HP pools, though. If you can make sure that the enemy isn't starting out in fireball formation, then they'll have more HP to go into the fight with, and eldritch blast won't seem so overpowering in context.
Basically, if your average combat is "a bunch of high-threat monsters, less a fireball's worth of HP", it's not surprising that it gets swingy.