As your players get more experienced, they get very good at destroying the monsters you set in front of them. Unless it’s a surprise attack, players are great at identifying the main targets and using their swords and spells to take them out with brutal efficiency. Had you an ettin as the main monster of the combat? Unfortunately, it didn’t get even one chance to attack after the wizards and warriors were through with it! Even when you have that one round of surprising your players with an attack, it’s likely they’ll regroup and defeat the monsters in pretty quick time.
This isn’t that much of a problem. You want the players working together well, and having a Total Party Kill (TPK) every other combat is a bit problematic when you’re trying to run an ongoing campaign.
The challenge is to make the important combats more interesting. A typical way of doing this is to change the terrain so the players can’t manoeuvre into the right positions, or are in danger from fiery pits of lava. However, terrain tends to not work quite as well in Theatre of the Mind games, which is how most I run most of my games these days. A patch of difficult terrain doesn’t have quite the same impact when you’re handling everything abstractly. Objects that the players can choose to interact with? Sure. But hindrances don’t work as well.
I’m finding that, in this edition, an easy way of making combat more interesting and challenging is to have a second wave of enemies attack in the middle of the combat. They throw off all the calculations the players have made. All of a sudden, the wizard wishes he or she had kept that fireball one more round or the fighter is caught out-of-position and can’t return to protect the magic-users who are being attacked from behind. And it throws a real threat into the battle.
There’s a battle in The Rise of Tiamat which works that way, with a pair of commoner guards suddenly being reinforced by a mage, a knight, several dragon cultists, and eventually an adult green dragon! You can run a lot of battles in Princes of the Apocalypse the same way, with reinforcements coming as the first battle is far from over. One of the most relentless examples of waves of enemies comes in the old Gary Gygax adventure, The Forgotten Temple of Tharizdun, where wave after wave of humanoids spill from the temple until finally a giant arrives and the players need to work out if they retreat or just keep fighting.
Every so often, having the reinforcing monsters not necessarily be on the first group’s side can be fun; and they attack both sides with glee. I’ve seen this happen where the players are the third side from time to time; not that often where your fight with goblins is broken up by chain devils who just want to kill everything!
Having the monsters approach from a different direction (or from behind the party) can throw a spanner in your players’ plans. This tends to work better when everyone has a good idea of the terrain. There’s a maze in Mike Mearls’s 3E adventure The Three Faces of Evil, which I drew out on a battle grid. Once the players had placed their miniatures, kenku attacked from everywhere – running back and forward through the maze, and attacking the party from all directions. This was one of those battles where Theatre of the Mind wouldn’t have worked so well; the battle grid improved it significantly.
There are also times when the players will cause additional monsters to enter the fight themselves. The party got attacked and then split up as they pursued the foes. The pursuit continued through multiple rooms, in different directions. This continued until they’d alerted an entire complex of separate rooms, which normally wouldn’t have known that battle was occurring nearby. The party found themselves fighting everything at once – and not all together! That’s rare though: most parties aren’t that stupid!
Probably the biggest challenge when determining when extra monsters enter the fight is determining when exactly they enter. I’m sure you’ve seen adventures that state, “On the fifth round of combat, the dragon arrives” or some such wording, only for the party to eliminate the original monsters in the third round! There are two ways of handling this. The first is the narrative way where you adjust how long it takes for more monsters to get there based on how the first combat is going. If the players are having a lot of trouble, delay the dragon attack. If the players are winning easily, move it up. The second method just keeps to the values stated in the adventure (is that more realistic?) and let the dice fall where they will. Being an impartial DM can lead to interesting situations, although, these days, I tend to be more interested in keeping the story functioning than being ruled by the dice.
One trick I have used on occasion is to surprise the party with the second wave if they finish the first early. How does this happen? Well, the combat is over, and various cries come from my players: “I loot the room!”, “I cast cure wounds on my friends!” and “I drink five healing potions!” Right, if they’re so busy doing that, they’re distracted and don’t get to see the monsters coming. Surprise!
How difficult is an encounter where the monsters come in waves compared to just all being in the same room together? Is it more or less difficult? I’m not sure of the answer. I think it depends on the situation. If the initial force isn’t that strong, it’s an easier battle than if they players face everything at the start, as they’re not overwhelmed with numbers. However, if the party are already struggling and have expended a lot of resources in the initial fight, then the resulting battle gets more difficult. I guess the only way to be sure is to throw a lot of monsters at the player characters and see how they cope!
In any case, using multiple waves of enemies has become one of my favourite DM tricks in D&D; you can’t use it all the time, because the players expect it, but it can prove a wonderful change to the standard flow of combat.