This might be more conducive to post in the homebrew forum, but I'm looking for feedback on technicalities more than the idea itself.

 I'm thinking of having one of my groups encounter a shield wall soon. The thing is, D&D combat isn't super conducive to that kind of coordinated battle. The easiest thing to do, I think, would be to make it rely on held actions. Per the DMG, a held action can be either an attack, or the (N)PC's full movement. So, my current thoughts are to make each of the enemies in the wall hold their action until the last one in initiative, and then they all march in formation. Any character directly in their path would have to make a Dexterity check or something for every ten feet they are pushed or be buffeted by the shields, taking bludgeoning damage. As long as they are faced head-on, the shield wall would have a +3 bonus to AC from the shields rather than the typical +2. There would be caltrops or ball bearings on either side of the wall so it wouldn't be quite as easy to circle around behind them, as well as archers in the rear, also holding actions for anyone trying to sneak around. 

 Is this "legal" per the rules of combat without homebrewing too much of it? I think it would be, but I would appreciate feedback from others. Also, does it sound like it would be more difficult than the enemy composition would suggest (currently thinking just MM duergar, slightly modified for shields and armor)? 

Technically, the rules don't allow for simultaneous movement. Even if you ready your action and say, I don't move until the guy next to me does, your readied action would trigger when the guy next to you starts to move, then you would get to move, then the guy next to you would complete his turn. That said, when I DM, I usually have identical creatures go on the same initiative count, so if you do something like that, they could kind of all be moving at once. But allowing them to move as a unit is homebrew. 

As far as pushing someone back, generally, you can't force movement with your shield unless you have the shield master feat (even then, you can only push it back five feet or knock it prone, by RAW. And there's nothing that allow the shields to do damage (unless there's a magic item, maybe?)). And even then, you can only do it as a bonus action after you have taken the attack action on your turn (which, if you are going the readied action route, would not be possible, since you are planning on using it to move). So it would be a lot to let them move, and push someone without an attack in there somewhere as well and do damage (and really tough if you want to do it as readied actions). Also, shove is not a save, its a contested check, pushers athletics vs. push-ees athletics or acrobatics. All that said, as they are NPCs, they don't have to follow PC rules, and you could certainly put something together as an NPC ability, but you're definitely in homebrew territory.

How difficult it will be is going to depend on the party composition. A wizard with a lightning bolt (or aganazzars scorcher, or a number of other spells) or an arcane archer with piercing arrow prepared would love for all the bad guys to be in a nice straight line like that. Or if the wizard has a fireball instead, they could just wreck those archers in the back.  Basically, most AoE spell users will really enjoy such a tight formation. Someone with Polearm master and sentinel would be able to break a hole in the line easily (stopping one of them from moving forward), then it would be up to you to decide if the rest have the discipline to stop or what would happen. But there are other characters who would be completely flummoxed.

I don't think you need to stretch the rules much to do this. It's not uncommon for DMs to have all mobs of the same type share initiative anyway, so this would be a perfect case for this. The rules may not technically allow it, but armies train to move in unison all the time, it's called marching. Of course in the real world it's triggered by either an officer calling out a command, a drum, or a trumpet. That's how you get everyone to move at once. So use that, it will add some really cool flavor to your battle if they hear the "Forward March" order go off, and then hear the drums as the shield wall moves towards them. And it's almost a sure thing that they will have started marching long before they engaged in battle. You don't start the march right in front of the enemy, part of the point of marching is too move large amounts of people in an organized way.

Just use the existing rules for shoving a creature (p. 195 PHB), they don't even really have to hold their attack, you can after all move, attack and then move. So when they come to someone in front of them, the shove, if the person is shoved they all move together. It should be pointed out that these sorts of formations weren't very fast once combat began, precisely because they were trained to not move further forward than the people on either side of them.

That being said, if your party has spell casters, probably they will be able to break the formation apart. There's a reason you don't see phalanxes and cannons in the same battle. Even so, I think this would be a really cool idea. It could make a really neat encounter, even if your players blow through the middle of it.

Quote from GadgetGirlGames >>

I don't think you need to stretch the rules much to do this. It's not uncommon for DMs to have all mobs of the same type share initiative anyway, so this would be a perfect case for this. The rules may not technically allow it, but armies train to move in unison all the time, it's called marching. 

Just use the existing rules for shoving a creature (p. 195 PHB), they don't even really have to hold their attack, you can after all move, attack and then move. So when they come to someone in front of them, the shove, if the person is shoved they all move together. It should be pointed out that these sorts of formations weren't very fast once combat began, precisely because they were trained to not move further forward than the people on either side of them.

 

  Yeah. I was thinking of it as similar to a mount's or elephant's charge/trample attack. I also think it might be easier to have them operate as the same entity until it gets broken down enough, at which point they would all act on their own turns. 

 Thank you both for your feedback.

You could try to modify swarm rules. Treat it as one really big, weirdly shaped creature. 

Yeah I'd try this as one creature with a "front" that could rearrange the wall on its turn, maybe using up some movement. I'd also just grant 3/4 cover by the wall rather than the +3 to AC. I think the interesting thing here is to figure out how to get around the wall, so a frontal attack should be discouraging. And letting them shift it keeps the battle dynamic.

I am going to steal this idea because it sounds like a lot of fun to run!