Herd-Hitting Attack

You're an adventurer, and you've joined up with other adventurers to take on a difficult challenge. The thing is, everyone's picked up the habit of standing around in a tight group. You're doing this to make it easier for healers to reach everyone and maybe pick up a Status Buff aura or two. But the enemy has noticed this and decided to do something about it.

A Herd-Hitting Attack turns standing in a group from a benefit into a liability. This could involve:
 * an attack involving an Area of Effect (AOE), like an explosion (the most common), or an attack that "rains down" or "spreads" over an area. Sometimes the AOE is a melee attack that hits multiple targets in a circle around the attacker.
 * an attack that "chains": after hitting the first party member, it will hit a second that's within a certain range of the first, and then a third in range of the second, and so on for a number of repetitions.
 * placing a status effect on one or more characters that causes damage or other debilitating effects to any other party members in a certain range, forcing the afflicted to run away from the party before they kill everyone. A particularly nasty variant has the status effect spread to anyone it hits.

In every case, spreading everyone out will reduce the potential damage of the attack, but make it harder to cover everyone. Note that not all area of effect attacks are Herd Hitting Attacks: if a point blank (centered on the user) or ground targeted area of effect attack is telegraphed (such as an obvious casting pose or Invocation), it's possible for the entire group to avoid the attack by moving out of the way without spreading out.

Commonly used by MMORPG bosses, though the occasional Mook might have one. Also appears in RTS games, possibly as a deterrent to the Zerg Rush. If the attack is the status effect variant and causes the afflicted to explode, it's Why Am I Ticking?

Video Games

 * Starcraft has quite a bit of this. The Science Vessel can use Irradiate on enemy units, causing damage to them and any nearby units until the afflicted unit dies or the effect expires, the Mutalisk can chain attack up to three enemies, the Devourer's attack causes a damage effect that splashes to nearby airborne units, the Valkyrie's rockets scatter around nearby enemies, and the Reaver, the Infested Terran, the Firebat and the Siege Tank in siege mode all have area-of-effect attack -- and in fact, the Firebat is a great defense against a Zerg Rush thanks to its powerful attack, its area of effect and its ready availability. The Arbiter unit has an AOE debuff which freezes everything in a radius, taking it out of the battle. The Protoss Corsair is an interesting example; it can only attack air-to-air, and its rapid-fire attack has an AOE radius so small it requires several units to be basically on the same spot in order to hit more than one; rather conveniently, air units in Starcraft can pass through each other and tend to bunch up on a single spot when given an attack order, so swarms of enemy flyers will generally form clumps at a single point.
 * War Craft III's Farseer hero can cast Chain Lightning, a chain attack that hits up to five enemies. The Naga Sea Witch's Forked Lightning can hit three. Archmage's Blizzard, Blood Mage's Flame Strike and Pit Lord's Rain of Fire can damage a targeted area. The Huntress can hit multiple targets with her targets. Siege weapons deal splash damage. So on...
 * Bloodline Champions has quite a large amount of these. Needless to say, avoid clumping up.
 * Runescape has a bunch:
 * Ancient Burst and Barrage spells strike up to eight targets adjacent to the spell's primary target.
 * The dragon halberd's Sweep attack strikes up to three targets in front of the wielder.
 * The rune throwing axe chains between closely-grouped targets.
 * Throwable chinchompas explode on impact, doing splash damage.
 * World of Warcraft:
 * The Shaman class has Chain Lightning like the Farseer in Warcraft III. Watch out for it in PVP.
 * The infamous Corrupted Blood debuff from Hakkar the Soulflayer is a prime example of the spreading debuff variant. When players found a way to carry the infection through the Boss Fight, they spread it all over the world, making it nearly impossible for low level players to survive being in cities or towns. Inspired the Zombie Infestation event.
 * C'Thun's Eye Beam and Sir Zeliek's Holy Wrath are unusual chain attacks: they increase in power with every bounce and only stop when there are no more party members in range that have not been hit.
 * In The Oculus the players get this ability when riding the red dragon, making it possible to deal extra damage to the boss by bouncing the spell off his allies.
 * Thaddius's Polarity Shift randomly gives each party member a positive or negative charge. Standing near players with the same charge gives a damage buff, while players with the opposite charge (or no charge at all) causes damage to the players, with both effects increasing in potency with the number of players in range.
 * Loken is one of the most difficult five-man bosses in Wrath of the Lich King because he both plays this trope straight and inverts it. During the main phase, an inverted area affect deals more damage the further you are away, so the group needs to cluster at his feet. However, he will occasionally cast a traditional AOE, dealing massive damage if you're nearby.
 * Every class has an AOE of some sort. Some have many. And sometimes bosses have their own versions (Deathwhisper's Death and Decay, Tyrande's Starshards).
 * The Death Knight's Pestilence ability is particularly nasty; when used, every Do T disease on the target is instantly spread to all other enemies within 10 yards. One of the DK's Unholy talents gives a huge boost to the damage of diseases spread this way.
 * A warlock's Seed of Corruption also deserves special mention - it's a debuff that functions as a bomb. When the target takes a sufficient level of damage (from any source), or dies, the bomb explodes and damages everything around the target (except the target itself, originally. This was later patched). Because group damage would cause the bombs to detonate nearly instantly, and the spell had no cooldown, this was - for a time - the most damaging Area of Effect spell in the game.
 * The Warrior's Whirlwind, the primary area of effect attack in the Fury tree, reduces its cooldown when it hits more than four enemies. It's not very efficient for small groups, but in larger packs, it can be deadly.
 * There are also a large number of abilities that invert this, healing people who stay together. The best known is likely the shaman's Chain Heal spell, but there's also Circle of Healing, Holy Nova (also a straight version, since it hurts enemies too) and so on. The only healing class that doesn't naturally get any of these is the paladin, and the Cataclysm expansion is expected to gve them a couple.
 * The strongest subversion of this probably comes with the various "meteor" attacks. While they may seem like either this or a regular area attack (depending on how much warning you get), they actually divide their total damage amongst everyone in the area: if everyone runs away, then anyone left behind will be killed. Notably, at least one boss has "meteor fists" which requires as many people as possible to stand in front of him, lest the tank be one-shot, inverting the standard cleave mechanic.
 * Some bosses put an AOE on a certain player, and force them to run away from others to avoid damaging the raid. For example, in the Omnitron Defense System encounter, Magmatron targets players with Flamethrower, doing damage to them and anyone else in the line of fire, and Electron can use chain lightning and put a debuff on players that causes electric damage to those standing near them. Players who cause wipes by not moving, and thus causing the damage to kill their fellow raiders, earn the ire of their groupmates and often get kicked out.
 * Bosses, often dragons, have cleaves, which hit people standing in front of them besides the tank (usually one other person, but it can be everyone in melee range). This prevents the entire raid from stacking and getting AOE healing
 * Everyone say it with me. BBBOOOONNNNEEEEEESSSSSTTTTOOOORRRRRMMMMMMM!
 * Fallout 3's Broken Steel DLC includes a level 30 perk called "Nuclear Anomaly". A player with this perk has a chance of suddenly releasing massive amounts of radiation, affecting friend and foe alike, when their health drops below a certain point.
 * Left 4 Dead 2 introduced the Spitter and its gobs of acidic spit to discourage the survivors from standing in a tight formation after skilled teams completed Left 4 Dead that way. There's even an achievement in VS mode for hitting all four survivors with one spit attack ("Great Expectorations").
 * The Charger's charge was also designed to break up teams, especially in narrow corridors. Just like with the Spitter, there is also an achievement ("Scattering Ram") for hitting the whole gang.
 * The Arc Beam from F.3.A.R.. If there are any additional enemies in the vicinity of your target, the beam will branch off and hit all of them.
 * Some Role Playing Games have a tradeoff, in that if you keep your party together you can unleash more powerful attacks, but the enemy can also more easily hit you with Area of Effect attacks. Examples include:
 * Shadow Hearts: Covenant
 * Wild Arms 4, Wild Arms 5 and Wild Arms XF.
 * Warhammer Online has Gorak the Ancient, who fires lightning bolts that do extensive damage on their own but whose damage increases considerably for each freindly target within a certain radius of the target, hitting BOTH targets with the improved-strength lightning bolt.
 * Civilization 4 uses artillery units to combat large stacks. In addition to hurting the unit they attack, they also deal some splash damage to other units in the stack as well.
 * In the first two installments, if a stack outside of a city or fortress is attacked and the defending unit loses, all units die.
 * Spells (and explosives) in the Myth series basically amount to this, with one of the most horrifying instances being Dispersal Dream, which will insta-kill most infantry units with a psychic explosion. Said explosion will then cause any adjacent survivor to do likewise, with the chain reaction continuing indefinitely. Tightly packed formation of 40 troops? *Boom* *boom* *boom* *boom* *boom*...
 * Supreme Commander has the Aeon's Salvation, a rapid-fire artillery (3 seconds reload) with the greatest range of any artillery in the game, that rains down small shells over the area of a football field. Great for turning bases into rubble. To balance it, it's freakin' expensive.
 * In City of Heroes, Blaster characters tend to specialize in these, though most of the other archetypes have some form of it, too.
 * Special mention should go to the Assault Rifle powerset which has Ao Es for five of the nine attacks (although one has a very small radius making it hard to hit multiple enemies)
 * The Robot-type Mastermind, in a very flashy way.
 * MAP attacks have been a staple of the Super Robot Wars series since the very beginning. Running into an enemy with one of these is especially nasty since most players tend to keep their units clustered together to take advantage of each other's Support abilities.
 * Tactical RPGs tend to feature these. Final Fantasy Tactics had, as a standard magic pattern, a 5-panel area. Disgaea even gives you the trade-off: you can get combo attacks if you stand near each other, but watch out for someone who wants to nuke the three panels you're all standing in..
 * League of Legends has a couple that qualify, with the most appropriate being Zilean's Time Bomb - for a bit of mana and a decent cooldown, Zil can set a target to explode after 4 seconds, dealing a large amount of damage to the target and any of his unfortunate friends standing nearby. For bonus fun with explosives, Zil also has a skill that partially reduces cooldowns (Rewind), allowing him to Bomb and then Re-Bomb a target, forcing the first bomb to explode immediately. While typically the relative power of the bombs decreases as the match progresses, they are quite deadly during the first 10 to 20 minutes.
 * Even more fun when the target doesn't notice the bomb and runs over to hug a friend. Zil likes it when his targets share the love.
 * And then there's Fiddlesticks and his CrowstorCAWCAWCAWCAWCAWCAWCAWCAW
 * Many, many spells, mostly elementalist spells, do this in Guild Wars. All the above variants exist. The rain-down variants were a particular Game Breaker in Player Versus Environment combat, until the AI was patched to not stay in a Rain of Fire spell for the full 10 seconds. When playing with AI allies, who will by default cluster around the party leader, careful micromanagement in needed to defeat certain bosses who can and will nuke an entire cluster of party members.
 * Psychonauts - Raz can get an upgrade to his psychic blast that causes it to chain to multiple targets.
 * Great for hitting the Den Mother's bombs and the Den Mother herself in a single attack.
 * Works pretty well vs the Confusion Rats in the Asylum Walls, too.
 * Ratchet Deadlocked has weapon mods that cause attacked enemies to send out electrical shocks to their nearby allies. These actually first appeared in Going Commando as hidden mods to certain weapons, but Deadlocked allowed you to add the mod to any weapon.
 * Tate combos in Shinobi, after which the enemies fall to pieces.
 * Seeing as every unit represents as many of those units as labelled, pretty much every attack in Heroes of Might and Magic (or Might and Magic: Heroes) hits a herd. However, there are multiple herd-hitting attacks - standard attacks of Cerberi, fireballs, larger fireballs, ring-area spells, chain lightning...
 * Eden Eternal has several dungeons with bosses that enjoy AO Eing the party to death. Usually those bosses are the annoying ones in the middle of the dungeon, and for a lot of them the best way to go is to have the one tank and two healers - one healer just to heal the tank, and the other healer to take care of the AOE damage. Another annoying trend of AO Es is to give the party a debuff that actually heals the boss! Did you just spend ten minutes getting his HP down? Too bad! Your tank was an idiot and now you have to kill the boss when it has full HP again.
 * In Team Fortress 2 this can be an effective tactic, since you need to capture specific points or push a cart, which by nature causes players to bunch up. The best ways to take them out is using a bunch of sticky bombs from a Demoman or sendinging in an ubered medic with either a Heavy, a Demo or Pyro. To avoid this, teams might try having a few people (especially Medics) stand just off the point, so they can move in if their teammates are killed.
 * Siege weapons in Age of Empires. One attack from a catapult can kill or at least damage most of the units in a formation. And they don't have to be enemy siege weapons either; you're not Friendly Fireproof from your siege weapons like you are from your archers or gunmen.

Literature

 * The MD Device from Ender's Game causes a Sphere of Destruction to spread from any ship that's hit--then any ship caught in the sphere will explode in a similar manner. A single, well-placed shot can destroy an entire fleet of starfighters.
 * Or, you know,
 * Joel Rosenberg's Hero. The sergeant's screaming at the soldiers to "Spread out! Spread out!" The colonel takes a different approach. "Nah, bunch up and save the enemy ammunition."
 * Nearly every Final Fantasy game has abilities or spells that can hit the enemy party as well as the player party. Some magic spells can toggle between single targets or multiple targets while others can only hit multiple targets.
 * Final Fantasy IX has a heard hitting status ailment in the form of Trouble. When a party member is afflicted with Trouble, any damage they receive transfers to the rest of the party by half of that damage taken.

Tabletop RPG

 * The Chain Lightning spell in Dungeons and Dragons. After hitting its primary target it arcs to the nearest target (losing 1d6 of damage), doing so again and again until it runs out of energy. AOE spells like Fireball also make standing around in groups rather a bad idea.
 * In 4e, Chain Lightning is a level 23 spell; it starts with 4d6 for the first target, then hit-or-miss it's a second attack for 2d6 for the next target, and then hit-or-miss on THAT it's 1d6 for every other enemy within 20 squares of you. And if you miss with any of those, it's still half damage.
 * There's also Furious Bolts, a spell available to Sorcerers who take the Lightning Fury paragon path. 2d8 damage to the first target, if you hit you do 2d4 to the next target, and you keep doing 2d4 damage until you miss or you run out of targets.
 * Lest we forget the cleave - great cleave feat chain, allowing you an additional attack each time you KO an opponent.
 * Hero Clix features this trope in the form of the power Energy Explosion, which in exchange for reducing a characters damage value to 1, it can cause damage to both the target of the attack and every character standing next to them. This can be real fun for characters who can shoot two or three targets at a time, since while the damage is reduced to 1, it also stacks.

Real Life

 * Most missile weapons can do this. But until World War I (the arrival of rapid-fire guns) it was not considered dangerous enough to compare with the danger of having formed troops plow through rabbles like a bulldozer. Plus the fact that formations are more easily commanded then swarms of individuals.
 * During World War II, Anti-Air defenses usually took the form of this, typically via various applications of More Dakka, in response to the advent of massed aerial attacks. It simply became more effective to fill the entire area with flak than it was to try and hit any particular airplane. Even so, against large enough attacks, this would take on shades of Point Defenseless without friendly air support.
 * This was also the preferred strategy of Luftwaffe fighter pilots against American heavy bomber formations in the same conflict: The Army Air Forces' battle doctrine was to field large forces of tightly grouped bombers, providing each other with overlapping fields of defensive fire while maximizing the number of bombs dropped on their targets. The Germans found it was most effective to simply charge through the formation head-on with guns blazing so that the defensive gunners wouldn't be have time to aim at them effectively. To make the most of this tactic, German fighters were often armed with powerful 20mm or 30mm cannons rather than the typical machine guns used by most fighters.
 * What grenades are design for, depending on what type is used, for both lethal and non lethal uses.