Almost every shooter has featured monsters with some specific gimmicks, ever since the genre was created. Most of the time, these gimmicks are designed to help the monster itself fit its particular role (or the gimmick is the sole reason the monster fits in the role). Some tanks might have some shield ability or armor; some swarmers might have some leap, dodge or wall-cling so they can swarm better; supports might be able to disable the player to benefit other monsters. There are two gimmicks in particular that I see often, usually to great effect: resurrecting and summoning monsters.
Resurrecting a monster is pretty obvious; there’s a dead monster on the ground, something happens to it, then it’s alive and able to attack the player again. Summoning (or spawning) a monster is where you create a monster that wasn’t there to begin with. The Archvile from Doom 2 is the most famous example of a resurrecting monster. While running around, if it comes across a dead monster, it could revive it with full health again, ready to attack you. Hilariously enough, the Archvile from Doom 3 is a good example of a summoning monster; as an alternate attack, it goes through a short animation where it spawns another monster, again ready to attack.
Resurrection is a classic, first featured in Doom, later used in Quake 2 (its purest form) and forms of it have appeared in Quake 1, Dead Space and others. When a resurrecting monster is sent along with the rest of the encounter, it’s basically a defensive support. Because of what it does along with other monsters, it demands your attention, otherwise you could end up fighting the same heavy-role, for example, twice in a row. You have to kill the resurrecting monster before the others or risk wasting resources. The Archvile does enough on its own without reviving other monsters to warrant singling out, but the Medic from Quake 2 or the Infestor from Dead Space have nothing going for them when it comes to combat ability. Yet in Quake 2 and Dead Space, when you see or hear one of them, you know you’ll target them first. A more major effect it has on the game, however, is that it lets the mapper refresh threats that the player has just defeated (with the added effect that you now have this other monster to deal with). Many times in custom Doom maps, I see situations where the player clears a difficult fight in order to access a switch or a key. After that, an Archvile spawns in and starts reviving things the player’s just killed. It forces him to fight through those same obstacles again, unless he’s fast enough to stop it before it gets too far (which he should know to do, if he knows what the Archvile does already). Of course, there’s also a random element as well, because there’s no guarantee that the Archvile will resurrect particular monsters, and it’s very unlikely it will resurrect all of them.
Summoning has a different, but similar effect. The main difference is that the player doesn’t necessarily have to have been through the area to get the same effect; he knows (potentially) that there is at least one monster there that is spawning other monsters. Like resurrecting monsters, it presents the player with a growing situation that he can sabotage if he moves quickly and understands the threat. There’s also potentially an even more random element as well, because the player can’t really predict how many it’ll have time to spawn or what monsters will be spawned, but that depends on how it’s done. Doom’s Pain Elemental creates only Lost Souls, and only does so when it can see the player; the Teleport Dropper creates a spread of random monsters, but again, only when the player is around; the Doom 3 Archvile can spawn them regardless, but it seems like it’s scripted on each Archvile which monsters it spawns. Because of this variation on how it can work, they might not be able to accomplish the same effects. A potential downside to summoning over resurrecting is that the player probably won’t be familiar with the situation he’s going into, and that may or may not be desired.
These gimmicks tend to have a decent amount of sub-gimmicks, as well. The definitions of these two systems are surprisingly broad, and there is a lot that can modify it. They could also be combined to a degree; Dead Space has a lot of corpses lying around, just as decorations to help the game’s atmosphere. After you’re introduced to the Infector, however, they become a bit more threatening. The Infector reanimates those corpses as new monsters, ready to attack. In very basic terms, it’s resurrection because of what it does, but it’s also summoning because it’s making something that isn’t a monster into a monster. Another point was brought up in Wolfenstein 2009; resurrecting a dead monster as something else. The Elite Guard enemy could revive SS troops as Despoiled; a similar, but faster, stronger and heartier enemy that can’t be similarly resurrected, itself. This has the same benefits of standard resurrection, only the player has a potentially new challenge to work with.
Another idea introduced in Quake is how to prevent these resurrection or summoning mechanics. The most basic form is preventing a monster from being revived if it’s gibbed (blown into pieces) by doing much more damage than it can take. The Zombie from Quake 1 didn’t have a lot of health, but it could resurrect itself after a few seconds if it wasn’t gibbed. This was later expanded in Quake 2, by allowing the player to gib corpses after they were dead (you had to do enough damage with the last shot, in Quake 1) so Medics couldn’t revive them. This gave you the choice of using ammo on a threat you’ve already put down to prevent it from getting back up. In my opinion, it didn’t work as well as it could have, because Medics were almost just as easy to kill as it is to gib a Gladiator, for example, that’s already dead. However, even more effective, to deal with the Infector, if the player is paranoid enough, he could chop limbs off of the corpses he comes across to handicap the monsters that will be created from them, or even prevent them from being resurrected. Summoning could be sabotaged as well. The Teleport Dropper from Quake 4 dropped these beacons that monsters could teleport to (it just spawned them, really), but those beacons could be destroyed. With an explosive weapon shooting the Teleport Dropper, you could easily kill it quickly and prevent it from summoning anything. This is a double-edged sword, as I’ll talk about later.
There is an aesthetic difference, also. When you summon a monster, you’re bringing in something that wasn’t there. When resurrecting, even with the gimmicks that I’ll discuss in a moment, you’re bringing in something that already exists into an environment and turning it into a threat. Take the Infector for example: it takes set piece decorations (that’s effectively what they are) scattered around the area and revives them as monsters. Story-wise, it’s just turning corpses into Necromorphs. Resurrection abilities give full context to what is going on, and in the case of the Infector and monsters like that, it animates non-threatening decorations as monsters. There are times when pulling a monster out of thin air is what you want to do, and also can have proper context, however. For example, the Pain Elemental in Doom 2 is very good at what it does and its summoning is given context because it’s very clear that the Pain Elemental is creating the Lost Souls it’s spitting at you.
As gimmicks, both of these are pretty tricky to balance. Not only do you have to consider the monster itself, but you have to consider the monsters they resurrect or summon. The Archvile in Doom was already a dangerous monster, but then it can revive Revenants, Barons of Hell and other dangerous monsters. However, monsters like the Cyberdemon and Spider Mastermind (bosses) couldn’t be revived at all. If they could have, that is a massive drain on the player’s ammo, at least, even if he’s never damaged by them. For summoning, Teleport Droppers could only bring in certain kinds of weaker monsters, the most dangerous of which was the Berserker (which was pretty dangerous, but rare, and you have some strong weapons by that point). The balancing factor for these gimmicks themselves is the monsters they effect; if too-strong monsters can be revived, the reviving monster is too strong. If too-strong monsters can be summoned, the summoning monster is too strong. This can be especially difficult to balance correctly for less-experienced players. People who know how to deal with reviving and summoning gimmicks know to go right for the monster with that gimmick, but other players might not and will attack the others first.
Of course, if the reviving/summoning monster itself is too weak, there’s the very real risk that the monster will be killed before it can do much of anything. This was the problem Teleport Droppers had. They were very easily killed before they can do anything, and because their summoning could be sabotaged they would almost never summon anything unless the player has no idea how to deal with them. Considering the build-up they Teleport Dropper is always given, it’s very underwhelming and ruins the effect for the player. These was a similar effect for Quake 2’s Medic, where killing the Medic itself was potentially much easier than shooting the corpses already lying around. Thus, you have to consider how quickly the monster can be brought down when you design it and whether or not the player would actually see its effect. I’ve also spoken a little on the glass cannon style of monsters before, and I may have mentioned that I’m not a fan of how those work, exactly. There’s a risk of doing that here, as well. If the Teleport Dropper, as easily-killed as it is, could have summoned monsters like the Light Tank (a very dangerous, hearty and hard-hitting monster), for example, that would make it a glass cannon. This is even worse for reviving and summoning monsters, as well, because the net effect is that either it’s pretty tedious for the player to keep killing these much-more-dangerous monsters, or the encounter you’ve planned out never actually happens.
Both of these concepts are classics, and have appeared in many different shooters over the decades. If the monsters are balanced correctly, they can be the most interesting, iconic or even feared monster in the game, and encounters with those monsters can be some of the most memorable in the game. However, to get to that point, you have to consider balance, how these are used, what these do, what you can use to modify them, and how they can go wrong. When in doubt, look to the examples we can find in these various shooters over the decades.