Swarmers are one of the more commonly-used roles, seen in most shooters even today. A basic definition is a monster that requires large numbers in order to do its job effectively. Most commonly, they’re weak monsters that have to mob you, or else they get picked off quickly and easily, but there are some other things to consider with design and balance.
The best example of a swarmer I can think of is the Cherub from Doom 3. Individually, they do low damage and don’t have much health, which you might think are the two important things. They’re also somewhat slow, but move in quick bursts; either leaping at you or evading an attack. However, this monster is almost always found in groups, in areas that are either wide with obstacles or somewhat cramped, allowing them to pin you against map geometry, or surround you.
The most fundamental aspect of a swarmer is its ability to attack in groups; not simply putting the player up against a lot of them, but some mechanic that allows them to attack (or otherwise do its job) all at once. Basically, everything about the Cherub screams “I do better in groups!” because they can’t do much on their own, but also because their evasion allows them to spread out to surround you, their small size means more can get up close at once. They don’t do much damage with individual attacks, but they can be tricky to dodge (especially with a group trying to claw you) and the damage really adds up. They are also really creepy-looking with disturbing sound-sets. Combined with their leap-and-claw behavior, that tries to bring a psychological aspect as well to make the player panic in the face of a swarm (not amazingly successful, in practice, but the intent is there).
The Manhack from Half-Life 2 is another swarmer, and one of my favorite monster designs to boot. It’s a small, floating robot with a sort of sawblade in the middle, and attacks just by ramming into you. Because it flies and is so small, it can attack you from different angles as you fight them off. It really shows that swarmers benefit quite a bit from being able to fly, so they can attack you from different angles. Somewhat like the Cherub, it announces its approach with a low buzzing sound of the blade spinning, but not in an incredibly obvious way or in a way that reveals their numbers.
Like the Cherub, the Manhack demonstrates that close-range (melee) swarmers really benefit from having some ability to get to the player very quickly. Cherubs have a leap attack, and Manhacks simply fly at the player quickly (and since they damage when they touch, they don’t have to waste time with an attack animation). By definition, they have to swarm you, and they can’t do that if they can’t close in on you. Usually quick bursts of movement or direct leaps work fine. However, in the case of the Manhack and Cherub, who are very small, they would be really difficult to hit if they were that fast all the time. The Cherub is pretty slow when not leaping and usually takes an obvious path when it leaps, while the Manhack, usually pretty fast, tends to ricochet off of walls (in a way that’s surprisingly cute for a killer buzzsaw) and take a few seconds to get its momentum back. These two let a quick player pick off individual members of the swarm after dodging an attack. You do want to give swarmers the ability to close in quickly, but in a way the player can deal with.
You might wonder why I’m not mentioning the aptly-named Swarmers from the Dead Space series, but I’m covering something very similar in the form of the Needle Bugs from Binary Domain (which allow me to make another point!). While they weren’t used in the game as often as I would’ve liked, Needle Bugs were very interesting swarm monsters. They were slow and died in a single shot, but they were tiny and usually attacked in swarms of at least a dozen, climbed over you, and periodically zapped you. The more that climbed on you, the slower you moved, and they invoked your pain animation every time they zapped you. They could be cleared off very easily by just using your melee attack a few times, but they were annoying, and what’s worse; they distracted you from other robots shooting at you. They’re a good example of an enemy that fits in both the swarmer and support role, and there’s a lot of potential for crossover there; supports that have a negligible effect, but have an additive effect with increased numbers.
Another interesting point about the Needle Bugs is how vulnerable and slow they are; they mostly depend on other monsters (not just their own numbers) to distract the player so they can get in and attack. I feel like this only works if the monsters has some support function, and/or does a large amount of damage if left alone (like Dead Space’s Swarmer). Otherwise, they’re so nonthreatening that the player doesn’t even have to bother with them until everything else is dead, then he can leisurely pick them off with his weakest weapon. I will talk about (relatively) nonthreatening enemies in another article, but swarmers are generally supposed to be dangerous in swarms.
Swarmers aren’t limited to melee-only attacks (though those are usually the easiest to pull off). The Drone from Halo shows all the characteristics of a swarmer: it’s small, fast, encountered in large groups and doesn’t do much damage per-attack. They can fly, but only in short bursts and usually cling to walls out of reach. Unlike other swarmers, though, they carry ranged weapons (weapons the player can use, no less). The details of ranged swarmers are different than melee, but the overall approach is the same; low-damage, dodge-able (but tricky to dodge) attacks from different directions.
I feel like swarmers with ranged attacks almost invariably have to fly or have some ability to get to high places, otherwise they could accidentally shoot each other and ruin their advantage of numbers. If you’ve ever played Doom and maneuvered a large amount of Imps into one single mass of monsters, you know exactly what I’m talking about; the ones in back and in the middle throw their fireballs, but just hit other imps and don’t do any damage to the player. To counteract this, ranged swarmers need some mechanic to attack all at once, like the Drone’s ability to fly and cling to walls. To be an effective swarmer, the ability to shift positions quickly is also pretty nice to have, otherwise they easily get picked off. Monsters with a generic hovering ability, as long as they have some mechanic to spread out, works as well.
Melee swarmers also have the habit of surrounding and mobbing you all at once, limiting the area the player has to move around. Ranged swarmers don’t try to (or at least don’t have to) get close in order to do their damage, so the player is more free to move around. In both cases, the player has to move or else get hurt, but with melee swarmers, the area you have is constantly changing and/or shrinking. That’s an advantage for them, but it comes with the disadvantage of actually being easier to kill as they get closer. Try to kill a Manhack at long range then try to kill one while it’s close up, and see the difference.
I did say earlier that swarmers need some ability to attack all at once, and here’s a good example of what happens when they can’t. Some would disagree with me that it doesn’t fit the role, but the Lost Soul from Doom (not 3, just 1 and 2) still does illustrate my point pretty well. Except for one horrible flaw, they were decent swarmers; they played no sound when they saw the player, and quickly rammed into him for damage. They were annoying to deal with, but that one crippling flaw was that one Lost Soul hit another, they would attack one another. The infighting in Doom is usually a great feature, but in the case of the Lost Soul it punishes a monster that wants to attack in swarms for attacking in swarms. If one at the back of a cluster charged at you and accidentally hit another, it took those two out of the fight because the victim would attack the attacker and they would try to kill each other. It also made the Pain Elemental (a support that only existed to throw Lost Souls at you) not as threatening as it could’ve been, because if it threw two at a row, it was very easy to get them to attack one another, or even get the Pain Elemental infighting. Beyond that, they were also a little too large and easy to hit. Aside from those two points, they worked really well; I’ve disabled infighting and reduced their size before, and they’ve been just as effective as Manhacks.
Lost Souls do bring one interesting element to the table; they’re sneaky. Because they don’t play a see-sound (like every other monster does), the player could walk into an area and be under attack before he knew they were there. They were also the smallest flying monster (though honestly, they were the same size as the player) which let them get around more easily. A good mapper could use these two elements to his advantage and surround the player before he can react.
The main difference between a swarm and a single threatening monster is that as you fight it, the threat lessens. A single threatening monster is just as dangerous when it’s almost dead as when it just enters combat (barring any low-health gimmicks); when you’re fighting a swarm, you’re constantly killing off individual members, reducing the potential damage you’re taking. It’s a difference a player might not immediately notice, but it is a difference can make an encounter more fun. An inexperienced player also tends to think of swarms as more threatening than a single strong monster, because the amount of monsters is a more easily-noticeable factor than amounts of health or damage.
Swarmers also do well combined with other roles. While swarmers can’t be tanks and tanks can’t be swarmers (they conflict by definition), a tank backing up a swarm can work very well. Doom 3, for example, uses Mancubi and Cherubs in tandem every once in a while. The Mancubus demands your attention with its bulk and damage, and gives Cherubs the opportunity to attack more easily. Supports and swarmers work pretty well as well, with the support handicapping you so the swarmers can attack even more effectively. In Half-Life 2, a few Poison Headcrabs combined with normal Headcrabs makes for very, very dangerous combinations, for example. The Needle Bug is also a good swarmer/support, making other enemies more dangerous through a mechanic enhanced by their sheer numbers.
Because they can also benefit from the element of surprise, scripting in a map can also help swarmers quite a bit. The Trite from Doom 3 is an incredible example of this; since it’s a small, spider-looking monster, it can climb out of anywhere, or drop down from ceilings on webs. It wasn’t very mobile besides that, however. It would’ve been great if it climbed on walls and leaped at you off of them to attack from different angles, for example, but as it was it had a tendency to get bottlenecked and end up attacking you one at a time, unless the area was open enough. As it stands, scripting helps the Manhack, as well, since they could appear from places the player can’t see and can attack from different angles, and the only warning was the tell-tale whirling.
Speaking of bottlenecking, the area in which swarmers are used also very important. If the area in which you fight them is open enough, non-flying melee swarmers probably aren’t going to pose much of a threat. If the area is a cramped hallway, swarmers of all types might not be able to take advantage of their numbers, so they’ll line up and attack one or two at a time, unless they’re small enough. You can either sculpt the area around the fight you intend, or try to sculpt the encounter around the surrounding area, but either works as long as you’re considering how the area will affect the fight. Melee swarmers and ranged swarmers can both exploit different kinds of areas, as well. Ranged swarmers usually want to attack in more open areas where they’ll all be able to attack from different angles (the first encounter with Drones in Halo 2, for example), while melee swarmers want to be able to pin the player against an obstacle of some kind (think of most of the Manhack fights in Half-Life 2).
If a swarmer is well-designed, situations involving them are more easily balanced by adding or removing some number of them. Because the main difficulty of a swarmer is specifically how many there are in an encounter, you can adjust the difficulty of the encounter by adjusting the amount in the swarm. This is especially useful in games like Doom and Quake that have different monsters or ammo placed on maps based on the difficulty level, rather than just damage-scaling. This is one of the characteristics of swarmers, though there could be other factors you can use as well if they have some other gimmick. For example, I once made a stealth-based swarmer where someone can customize their behavior when placed (i.e., whether or not it will try to be stealthy, etc.). The Cherub even has some code that implies it was going to be able to play dead, as well. As I’ve said just above, it also depends on the area they’re used in.
So now that these have been broken down into basic elements, you’ll probably agree swarmers are one of the most common roles we see in games (not limited to shooters; they’re common in strategy games, RPGs and other genres). Warhammer 40k: Space Marine, Binary Domain, Half-Life 2, Singularity, the Metroid Prime series and Red Faction: Armageddon are just a few examples of recent-ish shooters that use them. They’re relatively simple to design, simple to balance, and can provide the player with encounters that differ quite a bit from what other roles provide, especially when mixed with other roles.