A little DnD 5e idea I’ve been playing around with

Here’s a combo for you, which you who’ve been playing 5e probably already know, but might just have been overlooked:

Feats:
Polearm Master
Sentinel
plus
War Caster

Obviously the first two everyone knows are broken already, particularly when in combo.

Adding the third, however, to a HEXBLADE WARLOCK, (or Padlock if you have a few levels of paladin first), enables the following:

Abilities: you take a max-CHA build, I’m assuming you hit 20 early on here using an 18 + racial bonus. Adjust your figures if you choose otherwise.

Cantrips: Eldritch Blast (this is the reaction shiny) Booming Blade (for using your weapon)

Invocations: Agonizing Blast, Lifedrinker, Improved Pact Weapon, Eldritch Smite and some flavor ones

Let’s assume a pact glaive here, which you can summon to your hand any fight at all.

Some of this will be subject to a DM decision around “magical focus”. Since your Improved Pact Weapon is considered an arcane focus, and many abilities here read “when you hit” (not specifying a type of action, merely a hit), your DM may permit the use of effects on spell attacks cast through your focus, such as Lifedrinker and the Improved Pact Weapon magical bonus. As a DM, I do consider it valid, as the rule does not specify what form of action is being taken, only that you have to “hit” with it (meaning you roll a die to hit – so Magic Missile wouldn’t count here). You are rolling to hit with a magical attack or a physical attack, but your mileage with your DM may vary.

Back to the plan…

War Caster enables you to swap in a spell in exchange for the oppo attack – so long as it is a 1 action and targeting only that creature (thus no “Green Flame Blade”). Enter Eldritch Blast.

Now, your reaction instead of an oppo attack (which you can still do if you’d rather lock that guy down and kite him) becomes Eldritch Blast – and this feat triplet (when you finally have them together, probably 8th as a variant human, or 12th as anyone else) REALLY kicks in hard. Every reaction induced by this is now doing 2 attacks at 8th, 3 at 12th – each doing 1d10+11 (+5 Agonizing Blast invocation, +1 more for the Improved Pact Weapon bonus, and +5 necrotic more for Lifedrinker invocation). I’ll focus on 8th and 12th here.

Of course, you’re giving up that “speed set to zero” by doing this, but in exchange you are getting 2 or 3 attacks at 1d10+11 damage each. In addition, you have more warlock invocations you can use to further augment your EBlast with slow, push, or pull. Each hit will have an independent, additive effect on the target’s movement.

But let’s save two of those invocations, shall we? And one, let’s put it into Eldritch Smite, because there will be times when you just gotta put something on the ground. “Once per turn when you hit a creature with your pact weapon…” Again, not contingent on the type of action, only that you “hit” with it. That adds 4d8 plus prone (and possible falling damage if the target is flying) at 8th level, 5d8 at 12th.

It’s when you get physical that you can do this next part – in your regular attacks, you combine Booming Blade with your Eldritch Smite: all by itself, Booming Blade will add to your damage (scales up with level), and if the target voluntarily moves before the start of your next turn, it will take scaling damage when it does. Said damage is thunder, which is rarely resisted. You’re doing it with reach as well, using the pact glaive.

You now add Eldritch Smite after you establish the hit with Booming Blade, and use a spell slot to add 4d8 (8th level char) or 5d8 (12th level) damage to the hit and knock the target prone if it is Huge or smaller. Note that Eldritch Smite also references the “hit” mechanic and also does not specify whether that is with an attack action or casting of a spell.

So here, you use your main action with Booming Blade to injure the target for 1d10+1d8(scale)+4d8 or 5d8+11 to then drop them prone and add tell them “Stay!” with a conditional extra 2d8 (3d8 at char level 10) if they defy you and stand up.

If you simply want to beat them about the head and neck and don’t care about their movement, you use Thirsting Blade plus Polearm Master to generate up to 2 *(1d10+11) + 1d4+11 (pact glaive twice, and back-end. You have a myriad of potential triggers for your reaction, any one of which results in a lashing of either two or three sets of bolts for 1d10+11 with Eldritch Blast. If they are within 5′ reach you can make it the Booming Blade case above.

Of course, on top of these, using “Hex” and/or “Warlock’s Curse” is just extra 1d6 and Proficiency Bonus damage icing on that cake. If this is the BBEG of the adventure, of course both will be on, which also then turns this into a crit-fishing setup looking for 19s and 20s, which you would absolutely use Eldritch Smite on a crit.

Max damage figures at 12th level (assuming normal activity, no Eldritch Smites or other “nova round” tricks):
Main Action: Attack for 2 * 1d10+11, plus 1d4+11 : 46.5
Reaction: Eldritch Blast for 3 * 1d10+11 : 49.5 + possible movement effects

Total: 96 / turn (becomes 99/turn if you just spam Eldritch Blast)

Add 21 more if Hex is active on the target. Add 18 (8th level char) or 24 (12th level char) more if Warlock’s Curse is active on the target.

Note that this PC is not invulnerable – massed minions and/or ranged weapon/spell attacks are still a significant threat. However, from a “glass cannon” perspective, this feat combo has what it takes.