Unfortunately, while D&D has always catered well to the first type of encounter, making the second work well has been a problem. The DM can't just throw dozens of at-level monsters at the players without overwhelming them (or giving them some serious Deus Ex Machina) and due to level scaling mechanics, the can't simply use a horde of under-leveled creatures and expect them to provide any sort of challenge (due to them being unable to actually damage the PC). Fortunately, 4th Edition introduces a fantastic tool to work around cases where the DM wants to increase the body count while still keeping the encounter reasonable.
For those not familiar, minions are a special class of monster in 4e that serve as, essentially, cannon fodder. They are creatures with defenses and damage scaled to a specific level, but with one key weakness: they only have 1 HP, and go down immediately on any direct hit. Because their attack and defense are scaled at-level, they present a credible threat to the players and are still somewhat of a challenge to take down; however, because they go down so quickly, they're very satisfying to kill and not nearly as dangerous as a full monster of their level -- so long as the PCs don't ignore them!
But they die so easily! Why should I bother using minions?
The main strength of minions is that they provide an easy way to add bodies to an encounter without unbalancing it (for easier or harder). The most obvious reason you'd want to do this, as alluded to before, is to throw a horde of them at your players and create an altogether different flavor of encounter. Killing lots of things for their own sake is a perfectly valid reason to use them in an encounter, but there are more practical reasons you might want to add creatures as well.
Minions as Control
Consider your typical 8-by-8 room encounter in 4e. If you have a 5-player party, an even level encounter is going to have between 4 and 6 regular monsters (possibly fewer if you have elites or a solo). Depending on how much interactive/hindering terrain exists in the space, that leaves a lot of open area for the players to run around in that isn't necessarily all that interesting. This gives the players a lot of freedom to move around -- or to just huddle up in a corner and gang up on the monsters one by one. By trading one or two creatures for a half-dozen minions, you've now doubled the number of squares you can block and threaten in the room. Minions can be thought of as dynamic, persistent, movable area effect that can help to control the space and force the players to either divert their attention from the "main" foe, or surrender mobility.
Minions as Damage
If you look at the Monster Manual 3 On a Business Card, you will notice that brutes do a lot of damage and can soak up a lot of attacks, but have low defenses and so are easy to hit. Given enough minions with the right kind of abilities, a horde of minions can be just as dangerous as a single brute, while at the same time challenging the party to deal with it in an entirely different way (see below).
Minions as Defense
All monsters can be used as a "meat shield" to some extent; however, minions are specifically adept at this, because they can occupy a lot more physical space. This is especially powerful when combined with a "damage sharing/target switching" ability, such as the Goblin Hexer's "Lead from the Rear". This synergizes very well with minions used to control space, too.
Giving Different Players a Chance to Shine
Big monsters with a lot of hit points that do big damage may be cool, but they're also a Striker's bread and butter. It's perfectly fine to let the player have their fun and get to roll a dozen damage dice most of the time -- after all, that's probably why they picked the Striker to begin with. Sometimes, however, you need to let the Wizard or Warlord feel like a hero as well, and not just part of the Ranger's entourage. Minions accomplish this because, though they go down in one hit, they still require a full attack, and so can really slow down characters build to put out massive amounts of damage to single targets. Area effects, on the other hand, are tremendously effective against minions, because they go down even if the damage is not that great (so long as the attack hits -- minions take no damage from "half damage on miss" effects).
So how do I use minions effectively?
Aside from the above use cases, here's some specific advice for taking advantage of minion properties:
- DO gang up on players with minions. Their ability to surround and gain combat advantage via flanking is key to making minions do effective damage.
- DO use minions to shape the encounter on the fly. Since they're worth so little XP, you can throw two or three into an encounter to, for example, force that cowardly ranged PC out of the doorway and into the fray. Or if the players do rather poorly on a stealth check (for example), feel free to toss 4 at-level minions into the next encounter to boost the difficulty by one and give the feeling that the enemy was "prepared".
- DO feel free to use just one or two minions to "pad out" an existing encounter. Even if they go down quickly without doing any damage, you've tied up a PC for at least one turn while your main monsters go to work.
- DON'T clump the minions up too much, too early. Ganging up is useful, but one good AoE attack will wipe out your entire squad prematurely if you're not careful. Sometimes you should just roll with it -- the players deserve to feel like they've gotten the better of the DM on occasion -- but for the most part you want the minions to do at least a little damage or blocking before they all get knocked out.
- DON'T make it obvious that they're minions! Sometimes this is unavoidable, and players will usually figure it out into the encounter anyways; however, minions should play and feel like threatening monsters that happen to just not be as resilient as the other creatures on the board, not "giant rats in disguise" that should be ignored. Scattering them around with other creatures in the encounter is a good way to make them feel like a natural component.
- DON'T use a ton of minions on their own (unless your party lacks a lot of at-will AoE attacks). Minions are useful, but 20 of them does not necessarily make for a good encounter -- it will go too slowly, and they generally won't be able to do enough damage on their own to really threaten the players (only so many can surround a PC at one time, after all). If you want to use a lot of minions, consider mixing in a Controller or Artillery creature of some sort -- maybe even an Elite.
Hopefully these guidelines will help spark some creative minion usages out there! If I have the time, I hope to do a sequel article down the road about designing interesting minions from scratch, and some other miscellaneous minion tricks.