Named for the Zerg in Starcraft, whose main tactic is pretty much this in a nutshell — overwhelming numbers of cheap, disposable troops. (Memetic Mutation follows usage of this term with "Kekeke", the Korean equivalent of "hahaha.") Though as mentioned above, the meaning of the name in StarCraft multiplayer is rather different than the above description. In single player, the trope holds true for the Zerg.
The classic Zerg rush refers to using the zerg's advantage in the early game of being able to quickly churn out weak units (e.g zerglings) to sack the enemy's base before they can set up their slower-to-build but more powerful units. Of course, the numbers will be few, thus rendering the trope null and void in this case, but it is where the term originated. Even then, it is easy to stop or at least slow down by blocking your base entrance off with buildings... which has become standard even for bronze league players. The only way they can get through is if the terran player accidentally leaves the supply depots down.
The easiest way in Starcraft 2 to ensue a zerg rush is to use the nydus worm which can rush your troops from your base directly into the enemies. The drawbacks are the technology, the time it takes for the nydus to set up (and it's low health), and the slow rate of transfer through the worm (a unit every second) which wouldn't be bad if zerg didn't have naturally low health due to being fodder.
Strangely enough, in Starcraft II the Terran (with their dual build queue option) and Protoss (with their warp-in ability) can both pull this off better than the rather boom-y zerg. The classic Rush still tends to happen with Zergling focused builds due to them being dirt cheap with microscopic food requirements, both of which ensure that they are the only unit in SC2 that can be fielded in numbers approaching a hundred. Once these are made into Banelings things really start heating up.
In Starcraft parlance, a "rush" involves getting a particular unit as quickly as possible and attacking with it before the enemy could hope to have a counter. This applies even if the unit in question is a late-game unit. For example, a Terran player can do a Banshee rush (essentially helicopter gunships that have a researchable cloaking upgrade), and the Protoss can Void Ray Rush (airships that can Beam Spam). Both were possible tactics against Protoss and especially Zerg players, who often don't have enough early-game anti-air to stop such an attack unless they see it coming and prepare specifically for that (leaving them open to a more conventional ground attack).
In a rather hilarious bit of irony, the Zerg in SC2 are most effective when employing the opposite of the Zerg Rush in the early game. Instead of churning out a lot of zerglings early on, they instead constantly build Worker Unit drones far faster than the other races can, build a huge economic advantage, thenZerg Rush (Except at that point, it's usually just called a push or an attack). In fact, in pro-level play, it is up to the other races to hamper the Zerg early on to prevent them from "droning" and running away with an economic lead. If the Zerg does get into the late game, however, their armies tend to be massive and reinforcements easily replaced, allowing them to overwhelm an opponent through sheer numbers. While it is not termed a "rush", the effect to the layman makes it look exactly like what the Zerg do in cinematics.
In the custom game "Space Battle", each team (up to six members a team) has one capital ship per player. It is generally best to avoid going too deep in enemy territory and the best tactic is use broadsides and to quickly engage and disengage while farming resources and experience off the enemy's fighters. The problem is is that a player can use a cheat to log themselves in more than one slot for the game (which is cheating), but it allows them to control multiple capital ships (it is also possible to control capital ships of players that left the game, but by that point, they are heavily over matched by more upgraded ships). Since the player who does this gets enough resources for each ship like normal, they can just upgrade damage and armor and neglect the generally more important speed upgrades (used for disengaging and farming) and just rush the enemy with three or so more powerful, but slower capital ships. These players tend to get reported for this and any team that uses this in a tournament is disqualified.
The single-player campaign in Starcraft II Heart Of The Swarm adds a special ability that can make rushes easier, Zergling Reconstitution respawns ten destroyed Zerglings every 30 seconds for free. And another that lets you spawn three zerglings per egg at no extra cost, in about two seconds. The other option makes them do more damage, move faster, and able to jump cliffs.
Google has a game called Zerg Rush—search the term and little O's will crawl across the page and try to destroy the search results, and you have to click them to defeat them.
This is the favorite strategy of the enemies in Diablo 2 (even for the bigger guys). Think about it: You and up to 7 other guys, up against hundreds of demons. It especially gets nuts when you're up against those bug things, that spawn smaller bug things, from Act 2. There are some structures that spawn enemies, which look like something out of the Zerg Faction. I guess Blizzard LOVES this trope.
In World of Tanks a Zerg Rush is both a viable strategy and a suicide attempt. Either the enemy will be overwhelmed, or your team will be shredded as the entrenched tanks fire, sometimes blowing off a track, usually an instant death for the immobilized tank.In BT-5 rushes, at lower level maps you will always at least see five of them in a crowd speeding their way to enemy lines.
In Warcraft 2: The Tides of Darkness, it was a common (and much cursed) strategy of the Orcs to use a "Grunt Rush" to win battles — the father of the Zerg Rush. (Unlike Starcraft, you started with only 1 worker and no buildings. The thought was to build a Town Hall with the gold the game started you with to get an economy going. Some players, however, build a barracks instead and used whatever gold left to make basic fighting units and go attack the enemy, who would be lucky to even have a barracks started, much less have any units to defend with.)
Of course, this could only work on High or Medium resources. Those of us that prefer Low (where you only had enough for the town hall and first farm) had little worries of this sort of all-in. Though, more befitting the trope was producing footmen/grunts heavily out of three barracks and hitting your opponent when they were just starting to get knights/ogres, overwhelming them with the weaker infantry.
In Warcraft 3, the Undead have an explodingZerg Rush. This is because Necromancers casting "Raise Dead" raise two skeletons from every corpse - so if you send in a rush of ghouls backed up by a couple of Necromancers set to auto-cast "Raise Dead" the resultant explosion of skeletons from friendly and enemy corpses alike can be very destructive. Before the patch, some Alliance players built a town hall in front of the enemy town, and then swarm the enemy with an endless stream of militia.
Zerg Rushing is fairly common in Nintendo Wars, including the classic "Mech Rush" tactic and its infantry-and-artillery variant in the AW2 and AW:DS era. Even in situations where foot soldiers are ineffective, it is usually wise to deploy multiple cheap units rather than fewer, stronger ones (copters instead of bombers is a prime example).
Some COs have specializations that seem to have been designed with this trope in mind. Colin of the original Advance Wars series is the epitome of it, since his troops are weaker but cheaper. Hachi, Sasha, and Sensei are also particularly capable of using sheer numbers to overwhelm. Andy's supports this indirectly, as his repairs ability help all units a set amount, being more effective when you go for numbers over strength.
For most Fire Emblem games, this is a favored tactic of the AI opponents; they'll typically field armies that are anywhere between twice to four times the size of your party and, unless they're on the defensive, will send units to attack you in large numbers. This is offset somewhat by the player units having better stats, better equipment and the benefit of support relationships, so a properly-leveled party will take little/no damage from the resulting Rush. Hard/Maniac Modes, however...
For whatever reason, Anti-Zerg Rushing Scrubs are particularly common in the AoE community. Many games are played with a house rule that neither side can attack for some fixed length of time, sometimes ranging up to 45 minutes. It was so popular in the expansion "treaty" mode was introduced, so neither side could attack each other for 10, 20, 30, or 40 minutes depending on what is selected.
You can rush with Hittite elephants in . Much like real elephants they're hard to get rushing but man, once they start it's hard to get them to stop.
The Yamato cavalry rush was another staple of the original game.
Plus the late-game Shang villager horde, involving villager-only upgrades that turned them into passable fighting units. When you consider that the Shang had the cheapest villagers in the game...
Once the enemy AI runs out of military units, they still have all their Villagers out and about. Unless you've got to killing them, too, or even if you have, and if you attack a critical structure, such as their Town Center, they will often sic every Villager on you. At once.
For a dramatic demonstration of this, play with the "aegis" cheat activated. That cheat allows all players to create buildings and units instantly, but may also make the game damn near impossible to win as your opponents will inevitably send an endless stream of constantly-replenishing units at you.
Also, the dominant strategy is to flood out weak and cheap second tier units faster than the enemy, before gradually moving onto stronger units (the "flush").
In Age of Empires III, the Russian light infantry is weak and has low HP, but they're built by tens and are the cheapest units in the game.
In Overlord, your "Minions" are extremely expendable, and quite often, the easiest way to handle any given encounter, is to just keep throwing minions at it 'till it breaks. The sequel ramps it up further, as your primary antagonists will sometimes deploy their troops in shield-wall formations which are supposed to be unbreakable; you need to either kill the nearby commander or respawning spot, use mount-charges or siege weapons; otherwise, the formation WILL crush your forces. Unless you have a lot of patience, aren't afraid to personally wade hip-deep into a battle you are not likely to survive, and have a whole lot of extra minion Life Force to spend.
Alliance Of Valiant Arms has the "infection" and "battlegear begins" game modes, where the players have to fight off armies of zombies and killer robots, both of which use these tactics to try to overwhelm the players.
Over the years, a common phrase for beating endgame monsters is to "Throw Rangers/Black Mages/Summoners/Melees/Samurai/Dark Knights at it." Hell, the strategy is named Zerging.
Many a player can tell a story about the time they range-attacked a weak monster on the other side of an impassable obstacle, only to see the monster go charging off in some random direction... only to appear fifteen minutes later, having finally navigated the zone to find the player, and having alerted all its friends that it met along the way. Twenty floppy little bunny rabbits equals quick death.
One of the missions in the Crystalline Prophecy expansion involves 30 mandragoras attacking you in waves of about 5 or 6 each. They're comically weak and take an enhanced amount of damage, so it's part zerg rush and part whack-a-mole as the mandragoras die in one hit each. However, if you leave these enemies alone long enough they can Zerg Rush you by performing a move that takes nearly all of their HP and turns it into about 300ish damage. This attack can be used by the entire crowd in quick succession if you let them, which results in a near-instant and humiliating death on the player's part.
The mini-expansion which came out after Crystalline Prophecy, A Moogle Kupo d'Etat, features another such battle where a swarm of Cardians attack the player. They are exceptionally weak, much like the previous expansion's mandragoras, until you realize that half of the crowd attacking you are in the middle of casting some of the most powerful spells in the game.
There is a zerg rush as an encounter in the Zul'Farrak instance.
Many instances feature large packs of weak enemies that have to be killed by area of effect-attacks or they simply owerwhelm the players. Particularly notable are the ones like the boss encounter in Zul'Farrak where the enemies just spawn when an event is triggered and immediately attack the players.
The infamous Leeroy Jenkins incident. The dragon eggs in the particular room must be touched to hatch initially, but once they start hatching it usually results in a chain reaction which leads to entirely too many dragon hatchlings all heading towards the party at once....
On the Alliance there's the (in)famous Hogger raids. Forty level ones constantly rushing towards perhaps the lowest level elite in the game (level 11) results in some hilarious moments. The Horde does the same with Gamon, though he's not elite.
Basically the entire premise of Pikmin. The C-stick in the game is used to direct the mass of pikmin following you in a more precise direction, and when facing an enemy, is circled around to rush the entire pack in even faster. There's nothing more satisfying than swarming a tiny little Bulborb with all 100 of your minions from all sides.
This is the main tactic of the Free Drones, a faction of socialist proletariat who have a distrust for the well-educated upper echelons of society that once oppressed them (and so have a certain Dumb Is Good ethos). They feature an industry bonus (the citizenry being made up almost entirely of blue-collar workers) and a research penalty (...the citizenry being made up almost entirely of blue-collar workers), resulting in being able to deploy vast quantities of units but having subpar equipment. The Hive, the only other faction with an innate industry bonus, usually works similarly but to a lesser extent, mainly because they don't have a research penalty. They do have an economy penalty, though, which negatively effects their ability to research.
Another faction that likes to Zerg Rush is the Believers. They have a bonus to Support under their preferred political system which allows them to field larger armies, combined with a bonus to attack and lack of research that causes them to have lower-tech units then normal (albeit not exactly weaker so long as they strike first). They can't build as quickly as the Hive or the Drones, but they can maintain a larger army and their bonus to attack is incentive to strike first.
Another example closer to the Trope Namer is the planet's mind worms, who can and will come rushing out in ridiculous numbers if you're harming the planet in any way, with more joining the fray for every one that dies until you get your stuff together and up your planet rating, go and clean the fungus they come from completely, or simply get your entire land eaten by them. It helps that their Mind Rape attacks bypass armor.
The Brotherhood of Nod in Command & Conquer makes use of this at lower tech levels, able to produce huge numbers of cheap, expendable militia troops, as well as light, fast attack bikes, buggies, and tanks. However, while most soldiers fighting for Nod are poorly-trained, poorly-armed rabble, the other end of the spectrum is comprised of a much smaller group of super elites using technology that's often superior in many ways to that of GDI. If Nod has a single overarching approach to warfare it's not just zerging the enemy, it could probably best be described as sending favored sons to stab him in the back with a billion-dollar dagger made from alien technology while he's busy fending off the ragged but very fanatical mob in front of him.
The Scrin in Command & Conquer 3: Tiberium Wars can readily spam Disintegrators and buzzers while building an army of tripods in the background. And, since they're actually aliens, they are the real Zerg of this series. And let's not mention the mind-controlling cultists used by Traveler 59.
China, in Command & Conquer: Generals. Red Guards, the basic infantry, are built two at a time. Troop carriers come with 8 Red Guards free. To further encourage massing, groups of five or more of the same unit in close proximity get a damage bonus.
Terra Cotta Army wonder, a Zerg Rush kit, basically. Every thirty seconds (initially; it goes up by half a second for every infantry you control), you get a free basic infantry unit. Read that again. And, once you get the research (wonder?) that makes all timers complete instantly, you can basically send a never ending line of basic infantry trudging across the map towards your enemy. More like a Zerg Irressistable Force.
Also present in the game are the Chinese race, whose main bonus is instant villagers. Depending on Age, villagers can be upgraded to simple military units. This makes for a semi-effective anti Zerg Rush tactic, as a Chinese player with adequate resources can spam their city with villagers up to their population cap. Which can mean several hundred instant soldiers.
There is also the upgrade "Artificial Intelligence": All units are created instantaneosly, regardless of power or cost in resources. (Assuming you can pay, otherwise it doesn't work at all)
The Mordor faction in the Lord of the Rings: Battle For Middle EarthRTS is a prime example. Their basic unit is weak but free and comes in large groups. An even more extreme example is the Orc Labourer from the Isengard faction, an unarmoured orc wielding a woodcutter's axe. They each take up 1 command point, in a game where the command point cap is usually 300 at the very least.
This very much applies to the armies of Mordor (and to a slightly lesser extent Isengard) in the original novels as well. Sauron is practically the poster boy (poster-Eye?) for the 'plenty more where they came from' school of evil strategy. His Orcs are clumsy, cowardly fighters and only effective in huge numbers, especially against skilled warriors like (most of) the Fellowship.
Scout rushes are a frequently-suggested (if rarely-executed with more than 3 Scouts) strategy in Team Fortress 2—Scouts can reach the objective before any other class and have twice the capturing power at the cost of lower firepower and health.
The addition of the Pain Train for the Soldier and Demoman that gives them additional capturing power in exchange for increased vulnerability to bullets may start shifting the Metagame.
You can conceivably rush with any class, or any combination of classes, but some are only for comedy.
This video is a good example of how unsuccessful, yet humorous, a Scout rush would be.
Conversely, this video is an example of how successful, yet still humorous, a Scout rush can be.
An entire team "One classing" can effectively Zerg rush with almost any class and the right mix of load outs, particularly if they all stick together. Take for example the sniper, you wouldn't think it it would be able to effectively penetrate a teams defenses but most players forget the snipers secondary weapons are some of the best the game. With one out of every four snipers wielding the huntsman and jarate and the rest with SMGs and their choice of rifle, you pepper the enemy with an ungodly amount of fire that's backed by minicrits. If you all stick together you can even clear a hallway by having everybody pop out from around a corner and no scope fire at once minuteman style.
Another effective assault like this is to have a heavy rush. About half should use their mini guns and have sandwiches equipped to heal themselves and their teammates while the other half primarily uses shotguns to spy check and kill anyone who is trying the flank the mini gun wielders.
Yet another is take the idea of the double Medic strategy, where one medic Ubers another medic who in turn attacks with the uber saw to gain a quick uber than switch, and apply it to an entire team. Even without the reliance on constant invulnerably, a team of medics can be a deadly force because of their automatic primary weapons and their ability to heal each other, they can effectively form tow to three man fire teams where one medic will shoot while the other (two) heal that one, then trade off when they soak up enough damage.
Mann vs. Machine mode takes this to its logical conclusion, often sending a dozen or more of the same class at you all at once (and, in true Zerg fashion, it's usually Scouts). On some waves, it even sends endless hordes of Scout-bots until the main threat is dealt with.
In Total Annihilation, the equivalent tactic is the Flash Rush (or, inevitably, "Flush"): Arm's Flash light tank isn't quite the fastest or cheapest unit, but for its armor and firepower (dual energy machine guns that provide a slow but steady stream of damage, while also sounding awesomely like the Hyper Blaster from Quake II: the light laser of Core's equivalent unit, the Instigator, just isn't the same) it is very cost-effective and very brutal en masse. The default AI is vulnerable to rushes of any Tier 1 offensive unit besides the Commander even at the highest difficulty.
The Peewee Rush was even more brutally effective, but tended to crash the game due to having too many units on the screen...
This one isn't because of the number of the units, but the gun they fire. With a slight hex edit, the game supports 5000 units at a time. The problem is the sound the Energy Machine Gun (The Peewee's weapon) makes, and the way that Direct X 5 handles sound.
In Open Source remake - Spring - most mods still feature flash rush. Peewee rush is usually not as effective though - bigger maps and rebalanced stats mean that it won't reach the target before dying, unless their amount is really big. AoE units tend to deal with hordes of weak units in seconds, which reduces usefulness of this tactic. Peewees still have a role in the game, but it's not rushing.
World in Conflict has America being overrun on being essentially Soviet Zerg Rush. No missiles, just bunch of parachuting armies and war machines.
The first half of the campaign pits you against a Russian force generally 2 to 3 times your size; the first few missions pretty much end up in a total retreat.
In Warhammer Online, whichever of the two opposing realms (Destruction or Order) outnumbers the other is often accused of using this tactic to win in RvR, using their increased numbers and over abundance of tanks to steamroller the opposition. Trouble is, the tactic often does work if the underpopulated side can't put up a decent melee line to slow them down whilst their ranged take them apart.
In Dawn of War, Orks have an upgrade that allows them to get Slugga Boyz (their basic troopers) for free. This is fairly late game (as you need to already have most of your base built before the upgrade is even unlocked) but it allows the Ork player to fully embrace the concept of human wave tactics as wave upon wave of his boyz pour into the enemy base (in DoW you can have your units set to "auto-recruit", thereby allowing you to command your units without having to micro back to your base for reinforcements. Since Boyz now cost no resources other than head-count, this means a literal green tide).
Imperial guard have a glitch (or possibly a design feature, as it was never fixed whereby their morale upgrades increase their health. They can also take grenade launchers. You can have 140 guardsmen with more hit points each than space marines, 50 grenades, and all within the first five minutes of the game. Oh, and don't worry if they start dying; you have more.
One of the Event Matches in Super Smash Bros. Melee is called "Super Mario 128", where 128 smaller, weaker Marios swarm the field and you have to defeat every one of them. And just so you get the point of how weak Zerg Rush soldiers can be, these soldiers can be defeated with any attack in one hit. Even Luigi's taunt.
Darwinians, basic Virus units a.k.a. Virii and especially Multiwinians in the Darwinia series include such sheer number of units at disposal that they outnumber Zergs at least from eight to one during peak moments.
The coliseum in Tales of Vesperia uses this trope. You're forced to fight wave after wave of monsters, and it isn't too bad until you start fighting stronger Mooks that have the ability to stagger you. From there, you'll probably get staggered over and over and over again until you die. If this wasn't bad enough, bosses join the rush at set intervals.
Both your side, and the enemies' side can employ this trope in Final Fantasy XII: Revenant Wings. If you don't capture a summon gate quickly enough, then often you can end up practically wading through espers, in order to reach/capture it. On the other hand, if used against a level III esper (provided that most of the other espers have been taken care of), it can be quite helpful.
Star Wars Galactic Battlegrounds, since most of the differences between forces are in unique units and cosmetic changes, can let anyone do this. It's comparatively easy to sledgehammer a nearby opponent into the ground simply by hurling a swarm of basic troopers and mounted troopers at it. Of course, this can come back to bite you when everyone else upgrades tech levels first and curbstomps you with pummel siege engines and assault mechs.
The republic however get the ultimate Zerg Rush ability, They can put out troop units a lot faster then everyone else and their Tech tree is meant to send clone troopers to the field (Their tech gives you more food and better med droids to keep your men alive), the Rebels get slightly sturdier troops with decent anti armor to compensate for their lack of Zerg Rushing production and the Trade Feds have no housing required but lacks the resources to produce soldiers.
Left 4 Dead has this for the regular zombies. Whether the AI Director summons them or if a player gets vomited on by a Boomer, a huge swarm of zombies will all rush after the team, surround them, and proceed to beat the crap out of them. In VS mode, infected players may adopt the rush strategy by either having everyone attacking at once or rushing in after a Boomer player does his job.
Survival Crisis Z, oh man. Go to act 3 and find a neutral safehouse of level 11. You will never see the end of the mob.
Disgaea blatantly states this in the tutorial of the first game, saying the best strategy in the game is to rush one unit wildly with your soldiers. This is also an effective strategy for distractions, by sending out weak and useless characters, thus the AI auto targets the weakest link, leaving your main fighter several turns of beating the ever living hell out of the enemy.
Disgaea 3: Absence of Justice makes a reference to this after the first battle of the final chapter. After Mao smothers a Prinny bomb set by the brainwashed Vatos and Champloo helps them resist brainwashing relapse, a squad of brainwashed seniors appears to take down the group. The Vatos get a brief CMOA at this point by calling in their relatives for a diversion - all two hundred thousand of them!
Almaz: Heh... when you can't get good help, get more help...
Sapphire: Indeed. Numbers are power. Human wave tactics of this scale can only be called amazing.
Disgaea 4: A Promise Unforgotten features this in the God ending. So the party is strong enough to crush half a dozen of God's Parts? How about 8 million of them?
Serious Sam often has moments where rather weak Kleer skeletons or Marsh Hoppers can overwhelm the player just by having so many of them at the same time.
While Supreme Commander doesn't have a single faction that utilizes this tactic, the scope and scale of the game lets the best fulfillment of this, since the Arbitrary Headcount Limit is much higher than the usual FPS. One interesting (although ultimately doomed even against poorly placed defenses) strategy is to build 10+ factories with assisting engineers and pointing the freshly made robots and tanks towards the enemy base, sending a constant, never ending stream of units. This tactic can even work if you take the chance to send some siege-breaking units to destroy the front rows of enemy defenses, or use this stream as the distraction for a better localized attack.
Possible the best thing ever about Sup Com? Artillery rush! That is, building a continuing stream of artillery up to the enemy base and laugh with glee as his outer defenses are shredded to pieces by 50+ small artillery placements. Or better yet, if you can muster the resources, building 5 HEAVY artillery placements 10 Kms away from the enemy base and watch as the base simply vanishes by the 3rd or 4th salvo. Considering you manage to keep such a grand project hidden from your foe.
X-COM: Apocalypse has what's called the Hoverbike swarm, where you buy lots of cheap, weak, but highly evasive hoverbikes which you use to absolutely overwhelm attacking Flying Saucers. It works very efficiently for most of the game until the aliens start using Dimensional Multi-Bomb Launchers to take out many bikes in one shot.
Star Wars: Empire at War absolutely adores this trope. Bombers are fairly inexpensive, and have powerful weapons that bypass the enemy's shield. The downside is that they move slow and only come 3 to a squad. However, since EaW lets you drop reinforcements right next to your other units, you can drop 12 or 15 bombers essentially right on top of the enemy station in around 3 minutes, usually before the enemy has a chance to upgrade their space station.
Even more so in land battles.
The Egyptians in Age of Mythology are the ones with cheap weak troops that build fast. Throw in a few production speed upgrades and a Meteor god power dropped on a hostile chokepoint, and it's Wall of Slingers time, especially if you go with Ra and use your priests to empower military buildings. Isis boosts population cap and grants economic bonuses. This is very bad for whoever is on the receiving end. And Set has stronger slingers and the ability to summon cheap animals to fight.
The Norse fit, to a lesser extent. They can make their basic tier-1 warrior unit, the Ulfsark, right from the get-go from the town center. Also, Norse units gain Favour when they fight. These two factors combine to encourage aggressive early-game play; if you play Norse, it's a good idea to attack early and attack often, with lots of Ulfsarks. Doubly so if your patron god is Loki, because then you also get cheap heroes who can summon myth units. This trope is inverted by the Greeks, who build powerful, specialized and expensive units slowly.
The Titans expansion pack features the Automata myth unit; basically weak, cheap metal soldiers that build quickly. The only real advantage they have is that they can repair each other once the fighting stops. They can even resurrect their dead within a certain time limit. This can make them extremely effective when attacking isolated towns/armies: overwhelm, repair/resurrect, repeat. This is used against you in one of the campaign missions. Build Heroes. Lots of them.
In the same mission mentioned above, the main villain uses Zerg Rush against you by spawning Hell Gates. Except the Zerg in this case aren't weak. Not at all.
In Civilization III the Aztecs are made for this tactic. Their Jaguar Warrior unit is the earliest fast unit in the game, and fast units retreat at one health unless fighting other fast units. This allows for multi-turn rushes of epic proportions very early in the game. As a bonus, the Aztecs are Militaristic, which means that military buildings (such as Bunkers, which increase the total health of any unit produced in that city) cost half their normal price.
Also applicable to Civilization II when using the "Fundamentalism" government type - they can produce the "Fanatic" Unit that requires no upkeep or support and any reasonable size city can produce one a turn. If you have twenty cities in ten turns you can throw two hundred of them at your enemy.
This is Napoleon's favorite strategy when controlled by the AI in Civilization V. He usually builds up a sizable army made up of weak warriors and archers, then rushes you early on in the game. Because he spent most of his resources on this army, taking down his cities in a counter-attack becomes almost laughably easy if you are able to hold off his initial waves. Montezuma has a more useful variant, where he uses Jaguars, which are far less weak than regular warriors.
The Pikemen are already the weak-but-cheap alternative to swordmen, where even discounting their bonus against cavalry they are useful melee fodder since they don't require iron resources to build. The German Landsknecht unique unit has the exact same stats as the Pikeman, but at half the cost.
The Mohawk rush became infamous in the multiplayer. In a lot of cases (if the Iroquois player gets lucky with Ancient Ruins) it is undefendable regardless of how well the defending player plays.
Used by players in Guild Wars 2's World vs. World mode. Large groups of players storming keeps across the map are referred to as "zergs". This terminology has been all but officially adopted by players and developers alike.
In the Halo series, most skirmishes allow you to take on one squad of a few grunts/jackals backed by a brute/elite at a single time. When the drones show up, they go down easily and typically carry piss-weak weapons, but show up in really large groups. The flood also tends to send in wave after wave of infected (even more annoying in Halo 3, as flood infection forms can revive the combat forms you just put down).
The Husks and Thorian Creepers in Mass Effect 1. They're not very effective with it, except in the higher difficulties, however.
But in Mass Effect 2 the Husks are back with force, and in the tight, confined spaces they prefer to attack in, they will overwhelm you in moments unless you make very good use of your crowd-control abilities—even in Normal difficulty. On Insanity, they get armor as well, which severely negates the effectiveness of using knock moves to kill them.
This is also how geth hack as well; as a "platform" will often have over a hundred geth, they can just overload most firewalls.
Pre-genophage, the Krogan used this tactic.
The final cutscene battle in Mass Effect has Alliance capital ships doing a Zerg swarm against Sovereign. Despite Sovereign being vastly more powerful than any of the Alliance ships, it's eventually overwhelmed by sheer numbers and destroyed.
Mass Effect 3 trailers show Hundreds, maybe thousands of Reapers descending on earth. Averted/Inverted, as Reapers are Not cheap and Not weak, but still incredibly numerous
Conversely, this is the Alliance tactic to win back Earth in the finale — throw every vessel they, plus the considerable alliances you've spent the game building up, can beg, borrow, or steal to beat the crap out of the Reaper main fleet in an epic battle royale. If it hadn't been for the Crucible, it would have been a monumentally disastrous defeat.
Indeed, this turns out to be the final strategy for the final ground push against the Reaper transport beam to the Citadel. A massive forced of troops and armored vehicles rushing into an opening in the enemy defenses, hoping to close the gap before reinforcements arrive. Harbinger lands right in front of the objective and slaughters the entire force in seconds. Only Shepard and Anderson make it to their goal.
This is the strategy on both sides of the geth/quarian war in Mass Effect 3. (The quarian fleet also gets a few knocks in against a Reaper Destroyer this way via markerlight bombardment.) The end result, if Shepard is unable to broker peace, is that one side or the other is wiped out to the last ship and you have to pick which one.
Zerging a strong army with peasants in the Total War series is a viable strategy to wear them down. In Medieval II, when the Mongols and Timurids arrive, this becomes a very effective strategy, if only because once they arrive, the invaders have large armies but lack cities or castles to replace their casualties. You, meanwhile, can replace your losses, so you can just keep hurling armies at them to wear them down.
This is epitomized in the later Napoleon: Total War, due to the fact that muskets are deadly whichever way you look at it. Even against cavalry and cannons, a swift advance with full armies of militia will defeat most enemy armies. The only downside is morale, because Militia tend to break easily during combat (this is true for previous games as well), though this can easily be countered by a single expensive (though instantly recruited) general. Also, each militia unit that gains some experience will quickly become as good as inexperienced line infantry - without the exorbitant upkeep cost.
The Oda clan in the Shogun games recruit ashigaru (peasant) units cheaper than the other clans, and lend themselves naturally to this kind of playstyle. To add insult to injury, in Shogun 2 their ashigaru also have lower upkeep costs and stronger stats than the other clans' ashigaru. All-ashigaru armies (which you can field a lot of) is usually a valid tactic for the Oda.
Hilarious example in Total War Rome 2. Due to a typo in the game's coding, various sub-saharan African provinces had massive income, which resulted in in non-playable minor sub-saharan African factions being able to recruit and afford massive stacks of cheap levy troops, and then proceeding to steamroll historical powers like Egypt, Carthange, and occasionally even annihilating Rome in campaign mode.
Pretty much in any stage in Spore will the AI creatures, tribes, civilizations, and empires launch massive waves of enemies at you. This can be really irritating especially in creature stage, in which at most your pack can contain four other species while a single nest can contain 8-12 creatures (and there's a mod that adds even more).
Naturally, the best strategy in the tribal and civilization stages is to have sheer numbers over the enemies. This is especially easy to employ during the Civilization stage because land vehicles are rather cheap and sea vehicles only cost some 500 sporebucks more, making building an entire army very easy. Just hope that your that your machines are actually powerful enough to wage a war against a city.
Enemy empires (including the Grox) have no trouble being able to launch their massive space navies at your colonies.
A bug (or so we hope) in Panzer General 2 allowed the Red Army to buy the T-34 tank for free, thereby allowing you to fill the map with them and Zerg Rushing the vile Nazi.
The favored tactic of the Mastermind archetype in City of Villains. Though it varies depending on level and powerset, the average Mastermind can summon six minions to boss around. On Mastermind-heavy teams, upwards of 40 characters can be running around a map.
The mutants in Crackdown 2 employ this swarming tactic.
In the Homeworld verse, Vaygr strike craft squadrons have more units than their - individually stronger - Hiigaran equivalents. However, the Kadeshi are simply the kings of this trope.
Good thing they're really, really awful at defending their fuel supply
The Soviet Union pretty much solely rely on this tactic in Hearts of Iron II, especially in Human-vs-Human games where the Soviets enjoy five years of having to do nothing but build up their Industrial Capacity and then spam infantry/militia. The strategy can even compete against a talented Germany player's blitzkrieg tactics simply because they cannot replace the losses incurred fighting that many units spread over the entire European-Russian area.
The weapon of choice of the Swarm in Gratuitous Space Battles. The Swarm's ship hulls are noticably cheaper than their enemies' (which is everyone) but consequently their hulls, shields, and armor are also weaker. As a result, the Swarm can put a lot more ships on the field, especially in high-budget battles.
And of course there's Garden Gnome Carnage where seeming endless swarms of elves scale the sides of your building in an attempt to... give you presents?
This is a desperation tactic sometimes deployed in Dwarf Fortress, if whatever it is that's attacking a player's fort has wiped out the professional military (or turns up before there even is a professional military..) Dwarves have also been known to mass-stampede onto a battlefield on their own, not to attack, but to recover the clothing and armor of their dead compatriots.
Goblins are also prone to doing this, although given the number of traps the average DF player builds into a fort entrance, it rarely ends well for them.
In the FPSMMO PlanetSide a Zerg rush was usually necessary to effectively wedge the enemy out of a tower. Taking a base was no real pain, requiring a multi angled approach until the enemy could be booted out. Attacking one of the outlying towers however... wave after wave after wave of soldiers holding doors open, having rockets spammed inside before a sizeable group of power-armored infantry could rush the basement where the spawn room was...
Still popular in the sequel Planetside2, where sheer numbers can often carry a faction to victory in several bases in a row before their opponents can secure a base and hold them off with a really solid defense.
Some players of Steel Panthers are prone to do this: buying hordes of infantry (as opposed to a good infantry/armor mix), mortars (as opposed to howitzers) and cheap recoiless rifle jeeps (instead of tanks), even in open maps! The newest versions of this game have made spotting harder, which can make this trope more effective.
In the fighting game BlazBlue the character Arakune uses a sort of Zerg Rush strategy. God help you if Arakune curses you, because if he does he will summon a MASSIVE horde of bees and other insects to attack you. In fact, Continuum Shift gives you an Achievement for getting a 70-hit combo with Arakune, called "BEEEEES!!!!"
He doesn't even have to be on the screen to combo you: examples. Lots of curse combos last a long time on normal competitive matches.
Muv-Luv's BETA use that as their main tactics against the humans, and it usually proves to be very effective, since they outnumber the human forces on Earth at least 20 to 1, the average human pilot does not survive longer than 8 minutes into their first battle, and the BETA have control of the Moon and Mars. Plus it helps that there are 10^37 to 10^37+10^37x9^10 (It depends on how you interpret what The Superior says) BETA in the universe. As one player wrote in a stream of consciousness journal while they played the game for the first time, "BETA are zerg. Discuss." And then if that wasn't enough, the BETA are doing rushes with Ultralisk equivalents.
And in the Kohan games, the Ceyah (Undead) have the Zombie unit. These guys only cost a small amount of goal, have no upkeep cost, and can be made from the start of the game. The only limiting factor is population room. Throw in a Necromancer or a Kohan that can summon the dead, and you are screwed. Oh, did I mention that most of the Undead have damage resistances against ranged attacks? And don't get started with Shadelings. They are like Zombies, but have an upkeep cost of 1 stone per unit and are probably the fastest units in the game.
One of the selling point of Shining Force Neo is that the game enables over 100 monsters on the screen at the same time. When that actually happens to you, you're doomed.
In Backyard Monsters you could wait weeks to finally get the highest level non-champion monster and send like 12 in or you can send in 250 of the lowest level guys and watch them wreck crap. The beauty is that you're method of sending monsters in can't actually fire that many at once so you need to do it about three times and if you do the game will crash. So yeah you zerg rushed the base, the catapult, and the game itself.
EVE Online has this everywhere. Gods have mercy on your soul if you get caught in a big expensive ship by lots and lots of small cheap ones.
The big Alliances in player owned space typically like to jump in huge fleets of 300+ ships into other systems when invading, in order to overwhelm the enemy's defenses.
Section 8: Prejudice's AI is somewhat prone to doing this at times, especially in Assault games when the defenders go into Sudden Death. Subverted by the fact that they have the exact same stats as the player, although anyone with experience will have far better accuracy than they do. A fun game is to intentionally allow your team to go into Sudden Death and then stand outside your remaining Control Point shouting "THEY SHALL NOT PASS!!!!". The effectiveness of this strategy is debatable, but the awesomeness is undeniable.
No, seriously. They attack in their dozens, and on higher levels, the player can be firing full-auto into them with a machine gun and there are so many of them that the mass of zombies is still coming towards the player. At that point, you need More Dakka. Thankfully, there is the Pack-A-Punch machine for just that purpose... which wears off after the level and is expensive. There's a reason no one has beaten level 100.
Reason why Egyptians are broken in Seven Kingdoms II. Tactic? Build seat of power that not only increases Egyptian cities' reproduction rate but also allows you to conjure Isis, which give you instant boost to population, then build a few forts around cities and mass-conscript. The fact that Egyptian military units can use ranged attacks at literally level one doesn't help.
In Call of Duty: Black Ops, the level "SOG" throws pretty much the entire NVA at you. Endless hordes of AK-armed, respawning Vietnamese soldiers that charge past the Khe Sanh defenses and lob grenades into your trenches. There's a reason it's considered the hardest level in the game.
Trying to Zerg Rush in Age of Wonders II against the CPU opponents is not typically a good idea as the CPU will try to keep construction pace with your own army size. Additionally, the size and strength of the opposing force units is a major factor in the CPU determining what kind of threat and response level it will assume but it focuses heavily on quantity.
Generally the CPU will reliably fund three or four full stacks of units that stay together and defend its territory based around a Hero unit if given the chance to assemble the forces. This army descending upon you from out the Fog of War can be quite off-putting.
Galactic Civilizations: the AI has a soft spot for swarms of fighters and frigates. This tends to work well right up until the largest size of vehicle comes into play, at which point the path from [insert your homeworld] to [insert enemy homeworld] becomes littered with the husks of burning ships.
The Jagged Alliance 2 megamod v1.13 (named after the last official patch being 1.12) by default enables the "Drassen counterattack": if the player's team of mercs, I.M.P. characters (one-time-paid) and/or indigenous recruits take all three sectors of the city of Drassen, a cutscene plays which concludes with the Queen ordering an all-out attack to take the city, since it has both a valuable mine and an airport. In vanilla gameplay or with the option disabled fortunately she doesn't especially besiege the city although she may send several patrols to harass the town, but with the option enabled (as v1.13 by default does), she will instead mass them together — often at least sixty individual soldiers — for a simultaneous attack on what's usually three to five tired mercs without prep time and possibly a smattering of militia.
The game's combat being limited to twenty individual enemy soldiers in-sector at once means that the player's mercs/recruits and militia will only be facing twenty simultaneously, but that simply means that those first two soldiers are continually replaced with every death, as are their replacements.
Terraria's goblin army works like this. They're fairly weak, but there's at least a hundred of them every time they attack and their number goes up with every player that has enough maximum health, up to 12000+ goblins in the PC version with the maximum of 255 players on a single server. They're quite difficult when they first appear, afterwards they're a joke. The Frost Legion works similarly, but they're a LOT tougher. And then you have the Pirate Invasion and the Solar Eclipse monsters in Hardmode...
In The Legend of Zelda: Skyward Sword's finale, Ghirahim sets a horde of Bokoblins, Bulbins and Stalfos onto you to slow you down from preventing the resurrection of demise in the past. It is one of the most epic moments in the game. He also specifically mentions that they are not meant to kill or even stop Link, because they incapable of doing so, and are only useful to slow Link down.
Mega Man Star Force 3 has a multi-part minigame in which you try to fend off hundreds of cheap Omega-Xis clones for a certain amount of time. The mechanic is repeated during the endgame, but that's not a Zerg Rush so much as getting rid of random projectiles.
Pokémon Rumble Blast, in one of the types of battle, has you send all your Toy Pokémon (including the one you sent out) to take out a army of Toy Pokémon. Another one has you fight a bunch of Toy Pokémon trying to defeat your 3 Toy Pokémon and defeat the boss.
Evolva. Seriously, play this game and you'll be amazed at the great amount of numbers of enemies that attack you at the same time every single battle. Sometimes you may enter in combats against twenty enemies or so.
This tactic from an ancient version of League of Legends. Heimerdinger, who can place down turrets as an ability, prevents minions from moving through the middle lane by blocking their path with those turrets. Patched up ages ago; friendly units can freely move through Heimer turrets.
This is a commonly used tactic in Kingdoms Of Camelot on Facebook. The easiest way to take out an enemy city's defenses is with an initial wave of Militamen, the cheapest, most basic unit of the game. 'Scout bombs' are also used to destroy enemy scouting ability, sending a large wave of scouts to kill the opponent's scouts.
Crank your Aristocracy, Serfdom, Land and especially your Quantity sliders up in Europa Universalis 3 and you get this effect. All of these sliders make recruiting regiments cheaper; Land and Quantity also increase your manpower and forcelimits while Quantity also also increases the speed at which your regiments reinforce.
The Xenon and Kha'ak in the X-Universe tend to use this, swarming their enemies with individually weak fighters. The Kha'ak in particular like to deploy Clusters, a traveling mode Zerg Rush comprised of an M3 fighter and anywhere from five to nearly two dozen M5 scoutships.
In Muramasa: The Demon Blade, there's a late game mission that's basically a massive army of enemies charging after you endlessly with constantly replacing numbers, with the occasional massive enemy. You need to beat it with both characters to get the final endings.
Master of Orion: In the later stages of the first game, the AI loves to drop tens of thousands of ships on your head. Changes to the later games prevent fleets quite that large, but the AI does still tend towards believing that quantity has a quality all its own. Which is probably just as well, given their ship design philosophies.
In Jeff Wayne's War Of The Worlds this is a viable option for both sides. The Humans can quickly research armoured track layers, which require only a basic vehicle factory and are quite quick to build, and just steamroll the Martians with wave after wave of Tank Goodness. The Martians can do something similar with the quick-to-build scout machines, although it tends to stall against well-fortified Human sectors. The true Martian zerg rush is more tactical than strategic, utilising massed flying machines to rush a sector's HQ and blow it up (which instantly destroys all other buildings, neutralising the Humans' traditionally strong base defences) before mopping up any remaining units.
In Aquapazza, Multi and Ma-ryan have specials that summon a rush of robot maids and schoolgirls respectively.
Sins Of The Prophets, a Halo-based mod for Sins of a Solar Empire, follows its roots by having the UNSC need to outnumber the Covenant to win, further reinforced by the research topics available allowing it to outproduce the other.
In Clash Of Clans, a common tactic among players is to create a bunch of cheap units like Barbarians and/or Archers and unleash them all in a huge wave. Sometimes it works, sometimes it fails.
You are obligated to face these several times in the course of MechWarrior 4 and its spinoffs, especially Mercenaries. At least four times in that game, you are swarmed by over a dozen light units all at once, mostly vehicles or light 'Mechs, and they're dangerous enough to be a valid threat even when you have heavy and assault 'Mechs. Getting close to stomp on a vehicle as a One-Hit Kill is no easy task either, as Myrmidon, Quad Panzer, and Demolisher tanks have enough range and firepower to deal heavy damage, and missile carriers of all kinds can deal Death of a Thousand Cuts. There's even a late-game mission that pits your lance against a swarm of 32 vehicles, with nary a Herd Hitting Attack in sight.
Opponents in Genjuu Ryodan do this in all maps, with quick capture of mana crystals, summoning as much units as possible (with the occasional elite mooks mixed in) and rushing towards the player's units and castle as quickly as possible.
A favored tactic of the Mogeko. Most aren't very strong by themselves; however, they prefer chasing Yonaka in massive swarms, forcing her to flee through sheer force of numbers.
Employed often by EXALT in XCOM: Enemy Unknown. Unless you have seriously been neglecting your research, your XCOM troops will possess far superior armour and weapons to any of their operatives. However, between their fanaticism and willingness to sacrifice themselves, they will rush your position all in one go. If you use the right strategy, this most often results in the traitors on the receiving end of a Curb-Stomp Battle, but sometimes this actually works; after all, your squad can only kill so many EXALT Cannon Fodder per turn before the survivors rush around and flank them.
In the Fallout universe, expect Raiders to employ this tactic against you.
The New California Republic are prone to this. In a war against the much better equipped and trained Brotherhood of Steel note as a point of comparison: a NCR Trooper has a semi-automatic rifle (which may not be in the best nick given the kind of world the Fallout universe is), a combat knife and combat fatigues with a metal chestplate and leather shoulder pads; the average Brotherhood Knight has either a laser rifle or a kind of pneumatic sledgehammer, and Powered Armor is standard but were out numbered 10 to 1 , the NCR just threw waves of troopers at their positions and drowned them in bodies. The NCR won the war, by the way. Subverted in some cases: Rangers and Heavy Troopers are badass, and they have training and equipment to match, and even basic Troopers are more than a match for gangs and raiders so only small squads are needed to deal with them. And then there's...
Their main enemies, Caesar's Legion, who are even better at it. Many Legion grunts go into battle with nothing but improvised sports gear and crude cleavers. They started from a small tribe that conquered and subjugated their neighbours and in time have grown into a vast horde. Between their overwhelming numbers and sheer fanaticism (NCR Troopers have reported Legionnaires charging machine-gun posts head-on armed only with sharpened sticks and rocks), they have proven the gravest threat the republic has ever faced.
After initiating the Gala event in Dead Money hundreds of Ghost People come swarming up out of the sewers and maintenance tunnels, armed with spears, home made bombs and bear traps strapped to their fists. Unfortunately their huge numbers are backed up by their immunity to the deadly Cloud that blankets the Seirra Madre and their Healing Factor that brings them back from the dead over and over again unless you dismember or disintegrate them
The Quest for Glory series has a few examples, starting with the goblins in the first game. Though you only fight one goblin at a time normally, at the goblin training grounds they attack the hero in greater and greater numbers. Though they still line up and wait for the last goblin to be killed before they get their turn, with enough goblins they can prolong the fight enough to wear down your stamina, which can still kill you if it runs out in battle. Jackalman do the same thing in the second game, though in the AGD Fan Remake they actually avert Mook Chivalry and gang up on the Hero, having to be kicked away to avoid taking cheap shots from the side and behind.
In Metal Slug Defense, the descriptions for the basic Soldier and the Rebel Infantry advise this strategy, as they cost only 30 AP and have little delay between uses. They're too fragile to be any good fighting with their knives, but peharps a shower of their granades could bring good results?