Whack-a-lock

Character Concept

Planning

So the goal here is to be able to create zones using spells that we can then try to knock enemies into. At the same time, we are going to use Booming Blade to force them to decide whether they'd rather stay in the spell's effect or move out and take thunder damage from moving. We'll also try to use spells that cause their effect when a creature "enters" the effect on "a turn" or starts/ends their turn there. This gives us several opportunities to trigger the effect and means if we can do it simply by pushing them in, they don't need to willingly move there. Since we'll be using both spells and weapons, we have some limited options but none better than a Pact of the Blade Warlock. However, we do want some additional proficiencies and Weapon Masteries so a Fighter dip gives us a lot (plus CON save proficiency).

Level 1

It's usually a good idea if you're going to dip Fighter, to do it at first level to make sure you get all the best benefits. More HP, full armor proficiencies, CON save proficiency, and Weapon Masteries from level 1 are all awesome. We're actually not planning on wearing heavy armor because putting 15 points in STR is a big investment when we can just use CHA for attacking. But level 1 might be a little tough with low STR and relatively low DEX. Just be careful and things will get better by level 2. Here's my suggested starting ability scores:

STR: 8

DEX: 13

CON: 15 (+1)

INT: 8

WIS: 10

CHA: 15 (+2)

Take whatever proficiencies you want, but I suggest Acrobatics just in case you run into a grappling enemy. Take the Dueling Fighting Style for some extra damage. This isn't absolutely necessary, though. So Defense or Protection are also valid choices if you'd rather be more tanky or supporty respectively. We do get Second Wind for a little healing to help us stay alive until level 2. And most importantly we get medium armor proficiency, shield proficiency, and Weapon Masteries. There's a bit of options when it comes to Weapon Masteries but Warhammer is the one we need. I also like Whip for a bit of range in a pinch. Keep in mind, Pact of the Blade only works with melee weapons, though.

For species, there's no one best choice but I think Goliath (Hill Giant) is a very potent choice. It allows us to knock enemies prone at the same time as we knock them back, making it much harder for them to get back to us or out of the spell they land in. Human is a nice fallback option if you'd rather just have another Origin Feat.

As for background, take a custom one if you can so you can get everything you want. Ideally, get +2 CHA, +1 CON, 2 CHA-based skills, and the Alert feat. If custom backgrounds are not allowed at your table, just take Merchant. You get the ability score bonuses you want, one CHA skill, and Lucky which is still a nice feat.

Wear the best medium armor and shield you can get and try to use a ranged weapon or at least a whip for now.

Level 2

At level 2, things get much more interesting. We jump over to Warlock and get a lot for it. We get some cantrips. One of which should be Booming Blade. The other should probably be Eldritch Blast for cases where you can't get close to an enemy. For spells, there's nothing necessary but Armor of Agathys and Hellish Rebuke are both useful for a front-liner. The Invocation is what we're really interested in, though. As I mentioned, we really want Pact of the Blade so we can use CHA for weapon attacks. We can also summon either a Warhammer or Whip for just a bonus action if we don't happen to have a magical weapon we want to use.

We already have access to the basics of the build at this point. All we need to do is use Booming Blade on an enemy with our Warhammer. If they're Large or smaller (so most enemies), they'll get knocked back and have to move closer if they're a melee enemy.

Level 3

Things only get more fun from here on out. Warlock 2 gives us 2 more Invocations which are amazing. We're definitely not starved for options here. I think our best choices are probably Pact of the Chain and Repelling Blast (Booming Blade). Repelling Blast is very interesting here because in the previous rules it only applied to Eldritch Blast. This is still often the best choice. However, since we are planning on making copious use of Booming Blade and want to knock our enemies far away from us, this makes our knockback extra potent. Pact of the Chain allows us to get an imp familiar which can be invisible, is great at scouting, and can give us advantage on our Booming Blade attack in combat so we can hit more reliably. Also interesting note here: if you have a familiar, they roll their own initiative so you can use the Alert feat to swap initiative with them. Use this to your advantage.

If you don't like either of those (or if you have some other reliable source of advantage), other notable alternatives include Eldritch Mind, Lessons of the First Ones (to pick up one of the Origin Feats you don't have like Tough or Alert), or Otherworldly Leap (for some extra movement and easy positioning).

Level 4

We get a subclass at Warlock 3 and I think the best choice for this build is The Fiend. Extra temporary hit points, a great spell list which includes some spells that compliment our build, and some excellent defensive features really help round out our character. Let's not forget we also get 2nd level spells here. I think the must-haves for this build are Cloud of Daggers and Misty Step. Cloud of Daggers plays well with our strategy and Misty Step is just great utility.

So now we want to cast Cloud of Daggers on top of an enemy and whenever they move out of it, we try to knock them right back into it with a Booming Blade Push/Repelling Blast. We only want to use the Magic action to move the Cloud if we absolutely have to because it forces us to give up our Booming Blade attack. Generally, if I had to choose between the two, I'd probably pick Booming Blade over moving the Cloud because it would be more control and possibly more damage.

Level 5

This can be a rough level because other characters are getting Extra Attack or 3rd level spells here and we don't get either of those. However, I think we'll still do OK since our main goal is not damage but control and we can do that just fine with a cantrip. We do get an ASI/Feat here and I think our best option is Telekinetic. This gives us a few things that will really help. First, it rounds out our CHA to 18, giving us a +4 to our attacks and higher spell save DC (for when that comes into play). Second, it gives us something useful to do with our bonus action which, up until now, was not being heavily utilized. It also gives us the Mage Hand cantrip which is nice but probably more of a ribbon feature. The bonus action push will be particularly nice for moving enemies (or allies) into just the right position for us to knock them where we want them.

If you really want another option, War Caster is another good choice. We're planning on taking Eldritch Mind (if you haven't already) but there are other nice things in War Caster. We probably won't be making too much use of the Opportunity Attack because we're trying to knock enemies away. But if you are getting a lot of them at your table, this could be really fun for hitting enemies with Booming Blade on their turn and knocking them 20 feet away as a reaction. Definitely a fun option. The other reason to take War Caster is if you want to cast spells that have Somatic but NOT Material components (since we can cast spells with both using our Pact Weapon). It depends on how often this comes up in your games but very few of the spells I have selected for this build fit that requirement.

Level 6

I consider this the level where the base build kind of ends. We get 3rd level spells and 2 invocations. The 3rd level Fiend spells are perfect so the other spells we pick up are mostly for utility. We can drop Misty Step for Thunder Step unless you've been getting a lot of use from being able to teleport as a bonus action. I think that we don't really have enough pact slots to spare for it, but keep it if you feel the need. And also pick up Fly for additional utility.

But Fireball and Stinking Cloud from the Fiend spell list are going to be more useful. If you've been having difficulty keeping enemies inside the Cloud of Daggers due to its small area, Stinking Cloud might be a better option. It doesn't deal damage but if they fail their save, they practically lose their turn. Keep in mind it Heavily Obscures the area so it might be hard to hit enemies in it, but I don't think enemies will usually want to stay inside of the area. And if they do, just knock a bunch of enemies into it and blast them with a Fireball. It also only affects creatures that start their turn in the cloud, so feel free to move in and back out if you want to keep hitting enemies with Booming Blade. You'd both be blinded so the advantage and disadvantage cancel out.

As for invocations, again, there are a lot of options. Definitely take Eldritch Mind if you haven't yet (and you didn't take War Caster). We'll need to keep our concentration as reliably as possible. Between CON save proficiency and advantage, it should be rare that we lose concentration. Thirsting Blade is a good choice for situations where you may not want to use Booming Blade for some reason. Another really great option is Gaze of Two Minds if you want excellent scouting potential and also some potential for neat tricks in combat, casting spells from your invisible imp's location and having Devil's Sight (since your imp has it). Other invocations I mentioned previously are also decent options.

Level 7 - 20

From this point forward, continue taking levels in Warlock. Fill in any new invocations with whatever you feel like you need at the time. Previous suggestions still stand.

At Warlock 6, Dark One's Own Luck will practically guarantee we keep our concentration on our spells.

At Warlock 7, we pick up even more great area control spells to knock enemies into with Wall of Fire and Sickening Radiance.

At Warlock 8, we can finally round out our DEX by picking up Mage Slayer to get even better saves and better combat casters. If that's not a problem for you, just cap your CHA at 20.

At Warlock 9, Jallarzi’s Storm of Radiance is a fun option.

You get the idea. Grab as many zone control spells as you can and knock your enemies into them for fun and profit. Somewhere along the way, try to cap your CHA if you didn't at level 9.

Analysis

I'm really hoping I get a chance to play this build in a game. It's probably not the highest damage build out there but as most experienced players will tell you, the most useful spells control the battlefield and I think that is exactly what this build excels at. And on top of that, it is able to do it consistently and probably in just about every combat encounter.

The weakness of this build is that it lacks control against enemies that are Huge or Gargantuan. In my experience, you don't come across those sizes all that often. But if you do, you still have plenty of spells at your disposal with this build. I would suggest if you're going up against an ancient dragon that you probably want to hang back and lob spells instead of trying to get up on the front lines and smack it with your Warhammer. On the other hand, it's probably a bit easier to keep really large monsters in your area spells since they just need to have one square in the space to qualify. Either way, have fun with it.