(For the proper experience, run the music from this video while reading this page.)[Insert Fight Woosh here!]
Monster battles that spontaneously occur at random intervals while the player travels across an RPG. They are the same thing to video game RPGs that Construct Additional Pylons mechanics are to RTS games, Video Game Lives are to Platformers, and dying every ten minutes is to Sierraadventure games: practically synonymous with the genre, and annoying as often as not.
They were invented for tabletop RPGs and are reasonably common there. The original rationale was that as characters crossed a world map with each square representing half a day's march, they could reasonably expect to meet a pack of wild animals or band of highwaymen every few days or so (the practical reason was to get players Travelling at the Speed of Plot without obsessively checking behind literally every rock, shrub, and chair that they might encounter on the way). But nowadays it seems you can't walk ten feet down a narrow dungeon hallway without getting ambushed by a somewhat illogical combat encounter with nine mummy wizards.
The spontaneous generation of enemies is old hat in games in general, but RPGs are a special case: In the Tabletop Games, not every random encounter was automatically a combat encounter, as players could choose to interact with their encounter non-violently, depending on the individual encounter and the choices of the player and GM (sometimes, that NPC or monster really does just attack on sight). These aspects are only tenuously connected in many Western games, and wholly separate in most eastern ones, to the point that there are different screens for combat and everything else.
Typically, random encounters only occur on The Overworld or in dungeons. If you run into a random encounter inside a town, it's likely a sign that something has Gone Horribly Wrong (unless the town is already known to be a rough neighborhood).
Having to fetch little Timmy from the forest is a common sidequest, and while Timmy is generally menaced by the monster of the hour, he presumably went unnoticed by the scores of flesh-eating slugs and mobile venomous plants that harassed the player characters every 30 seconds as they went in to fetch him. (Clearly, little Timmy Took a Shortcut.)
Often, there will be an item, ability, or mode of transportation that affects the encounter rate. Upping this rate can be good for Level Grinding, while abilities that shut off random encounters entirely may save the player some annoyance in the short term, but can also deprive the player of much-needed experience points to strengthen their party for the next plot-motivated battle.
Often the breeding ground of both Goddamn Bats and Demonic Spiders, the former moreso (and, if you're lucky, Money Spiders). Typically preceded with a Fight Woosh and succeeded with a Fanfare, with Battle Theme Music playing throughout its duration.
The video game version is becoming something of a Discredited Trope nowadays, with fewer series playing it straight, and many of the big series dropping it in recent installments. It's still used, but it's not as universal as it used to be.
Subtrope of Random Event. Contrast Pre Existing Encounters, a specific aversion where enemies can be seen (and avoided) on the field, and Fairy Battle, a variation where the random encounter isn't hostile but actually helps the player along. See also Encounter Bait and Encounter Repellant for the mechanics of adjusting the rate of encounters, as well as Escape Battle Technique, for the mechanics of avoiding them once they've started. Compare Big Lipped Alligator Moment, where as that trope is about a single random occurrence that goes unmentioned, this trope covers repeating events that are rarely, if ever, mentioned by the story.
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Beat Em Up
In God Hand after you beat a Mook there's a chance a demon will spawn.
Parodied in 9-Volt's stage from WarioWare. You don't actually get to move around the Retraux RPG world, but each random minigame is introduced with the message "A game appears!"
In Purple, walking on blank nodes on the world map may randomly pit you in a battle with a demon.
Role Playing Game
Drakkhen was notorious for this. Moving around anywhere in the overworld, every few seconds you would get random encounters with exceedingly deadly monsters, which made navigating it and traveling between dungeons a royal pain in the ass. Hell, even if you were just standing still and minding your own beeswax, something might decide to jump out of nowhere and annihilate you.
In the Pokémon games, outdoor areas generally limit encounters to areas of tall grass, giving the player some ability to limit how often they have to fight a wild Pokémon; on the other hand, wild Pokémon can show up at any time when exploring underground caverns or surfing across bodies of water. It is also standard practice for shops to sell "Repel" items that will temporarily prevent encounters with lower-level Pokémon.
The fact that Repel items only ward off Mons weaker than yours also makes them useful for locating certain legendary Pokémon who randomly roam across the map, because they are noticeably higher-level than the Mons that inhabit the area.
Similar items exist in many other games, such as Star Ocean, including items that increase the chance of an encounter. The Dragon Quest series of games all have an ability or item (sometimes multiple ones) that instantly summoned a random encounter; handy when looking for Randomly Drops.
It's also worth mentioning that Random Encounters in Pokémon uniquely serve a purpose beyond grinding and the games wouldn't work without them (note Pokémon Colosseum, which had a rather lukewarm reception). In addition to providing experience points, random encounters also allow the player to further their overall goal of acquiring more of the eponymous creatures. Without the random encounters, the game wouldn't go anywhere.
Not true; Pokémon could very feasibly do Pre Existing Encounters using overworld sprites from slightly older games, and retooling the encounter mechanics. It's just weird at this point that we almost never see Pokémon in the wild outside of battle.
In the Baldur's Gate series, random encounters only occurred when transitioning between wilderness areas; all other battles were predictable. The voice dramatically intoning "you have been waylaid by enemies and must defend yourself" as a sort of Fight Woosh in the eight hour journeys between areas quickly became frustrating to the enterprising player.
While random encounters may have been toned down on the overmap, enemies in each area pretty much teleported in wherever the Fog of War covered them. This meant that if you stood around doing nothing, you wouldn't run into anyone, but if you walked back and forth (even into a sealed cul-de-sac) you would end up fighting infinite amounts of enemies (and generating truly absurd heaps of corpses).
Skies of Arcadia for the Dreamcast had extremely excessive amounts of random battles, a feature made all the more irritating by the fact that the theme of the game is supposed to be free-roaming exploration — rushing on deck every ten seconds to fight off wave after wave of sentient leaves and oversized tapeworms when you're looking for treasure or Discoveries kind of spoils the mood. There are even random battles in areas where the pressure and winds are supposed to be so great that the characters don't dare step outside the ship. The Director's Cut Updated Re-release for the Gamecube supposedly cut down on the amount of random battles, but many gamers still complained about them.
It should be noted the Skies of Arcadia not only had an item to reduce random encounters (called the White Map), but also a "Black Map" that increased them as well. One wonders why...
The Black Map had a special ability that prevented any enemies in battle from running away. Needless to say, this was essential if you wanted to fight Loopers. Other then that though....
However, it's somewhat averted later, once you get a special upgrade for your Cool Ship you can fly above or below the clouds, thus completely eliminating random encounters.
And in the Gamecube version you get the White Map shortly before you can skip encounters completely. Whee. On the Dreamcast version you get the White Map for finding all the discoveries, which means that you're probably near 100% completion of the game and you don't really need it anymore.
The Fallout series has random encounters while traveling. A good enough outdoorsman skill, along with a perk and a specific inventory item, increases the chance that, should you happen upon some enemies, you'll get a choice of whether to fight them or avoid them. It is not possible however to eliminate forced encounters entirely. Even a maxed out outdoorsman skill won't give the fight-or-avoid choice every time.
There are also Special Encounters, where something out-of-the-ordinary occurs (often not involving any battle at all). These are unavoidable, and will occur whenever triggered regardless of your outdoorsman skill. This exists in the original Fallout games, including the tactical simulator Fallout: Tactics. In addition to using your Outdoorsman skill to give the fight or ignore choice, your Luck stat indicates how often you'll get a Special Encounter over a random one.
Some players just roam the landscape for weeks until they have a specific Special Encounter that grants them uniqe items like the alien gun, one of the best energy weapons in the game.
Fallout 3 has a variation of this - there are multiple pre-determined points that can spawn random random and/or special encounters, and walking close enough to them will cause an encounter to be randomly picked off a list and spawn. The random encounters are typically attacks by raiders or mercenaries (which mercenaries you get depends on your Karma; Mister Burke will hire evil Talon Company mercs to assassinate a Good character, while the freelance cops called Regulators will take it upon themselves to put an Evil character down), while Special Encounters run the gamut from two wasters fighting over a refridgerator full of clean water to a flying saucer exploding overhead. This is a mixed bag, as some impressive equipment can be withheld from the player at random (the aforementioned UFO, for example, drops a unique laser pistol that sets the target on fire), or cause tough encounters to spawn very early (as anyone who's had the "wounded Deathclaw" spawn in front of the Super Duper Mart you visit around level 4 can tell you).
Fallout: New Vegas has forsaken random encounters entirely. While scripted encounters still exist, such as NCR or Legion death squads if either of them dislike you, they happen at fixed points. The weirder Special Encounters largely replace more mundane ones, and are accessed via the Wild Wasteland Trait (for example, Wild Wasteland replaces an ambush by some well-equipped Mercenaries with a battle with three crash-landed aliens).
Wasteland had random encounters almost everywhere, including cities. There are shortcut sewers whose whole function is to allow you to skip some encounters.
Breath of Fire II was so bad about this that it even had a little dancing imp in its pause screen to indicate the level of "monster activity" in the area (and a "smoke" item that supposedly reduced it).
In Tales of Phantasia, there were Random Encounters, but also one chapter where you'd have to storm through a lot of enemies through the maze-like Valhalla Plains. These enemies appeared on the field and chased you, and if they caught you, well, you'd have to fight them. Destiny and Eternia, similarly, used Random Encounters, but had some segments were you could chase or be chased around by something on the field.
Most Tales games offer items that allow you to increase or decrease the encounter rate.
Tales of Innocence exists halfway between random and pre-existing encounters. Enemies appear on screen beforehand, but they materialize randomly, often times directly on top of the player.
Sigma Star Saga for the GBA justifies this trope. Your allies have unmanned ships flying around above, and when they see something they don't like, they get spooked and summon up the nearest available pilot to help.
Superhero League of Hoboken has random encounters in every map, but if you win a fixed amount of battles in a map, you'll be informed that it's cleared (and you won't meet any more random encounters there).
Generally averted in The World Ends with You, in which you fight Noise by scanning your surroundings with the Player Pin and then touching color-coded monster icons to engage the battle. (Be warned: black Noise icons home in on the player when scanning!) However, near the end of the game, Reapers begin to spontaneously ambush and attack the player when travelling from one area to the next.
Wizardry VII did this so annoyingly, you could literally fight one battle, turn ninety degrees, and find yourself facing more monsters.
Wild ARMs 2 and Wild ARMs 3 had an interesting variation on this called "Migrant Points". Just before a random encounter, you would be alerted and given the chance to skip the battle by spending Migrant Points (which could be restored by fighting battles or picking up crystals). At higher levels, you could even skip low-level encounters for free. This system was also used in the remake of Wild ARMs, Alter Code F.
Wild ARMs 4 and Wild ARMs 5 allow you to turn off random encounters in a particular area after you've "cleared" a save point (usually by fighting a battle of some kind).
The Elder Scrolls: Arena featured extensive dungeon random encounters, such that enemies could spawn right in your face. In Daggerfall, a finite number of monsters spawned at preset locations in the different sections of a dungeon and random encounters were relegated to mostly disturbing a sleeping player.
There's a reason why MOTHER 1 is considered the black sheep in the MOTHER trilogy, and that's due largely in part to the numerous (almost never-ending) random encounters thrust upon the player. Combine that with generally being Nintendo Hard, and...
The random encounter rate in MOTHER 1 is ridiculous... when you didn't want to level up. You'd fight every two or three steps.
In the Web RPG AdventureQuest, every monster you fight is taken from the Random Encounter list. It doesn't make sense sometimes; you can end up fighting a Light Dragon in the Abyss.
The 7th Saga has a variant: you can see random encounters in a crystal ball located in the upper left of the screen, allowing you to dodge them in theory. In practice, they're so fast and numerous that you can't avoid them, and they move through walls to catch you.
Golden Sun used random encounters to an annoying degree, where you'd get into fights every few steps if your party's level was below or around the levels of the enemies. Being higher leveled reduces the encounter rate. Using a certain spell/item also helped reduced the encounter rate, depending on your level. One piece of equipment actually increases the encounter rate.
The piece of equipment in question, however, is incredibly useful near the end of the second game, because you will need to level-grind to beat the Bonus Bosses, and the best place for doing so has a below-average encounter rate. Annoyingly enough, it's only available in the first game, so people who threw it away going "the encounter rate is high enough, thank you very much" (or didn't transfer data at all) end up having to spend even more time doing so than everyone else.
To the dev's credit, they do usually turn off random encounters (or turn the rates down) in rooms with particularly difficult puzzles, which makes it a great deal less frustrating than it would have been. If the puzzle in question spans the entire dungeon *cough elemental rocks cough*, you're out of luck.
Lampshaded by Amiti in Golden Sun: Dark Dawn when The Luna Tower is activated and unleashes dark monsters on the world. He complains that the party could not walk five feet without being attacked.
Destiny of an Emperor has these in spades. There's an item called the Smoke Pot that prevents random encounters from happening, but it doesn't last very long, and you'll often have to cross vast expanses of overworld and dungeon between towns and fortresses.
In Kingdom Hearts, random encounters are justified by having The Heartless drawn irresistibly towards keyblades and their wielders. They are then subverted entirely, because on any given world, The Heartless always appear in exactly the same place every time. What kinds of Heartless appear, however, changes as you progress.
Dragon Age: Origins had an unconventional take on Random Encounters (which is ironically close to how random encounters are handled in tabletop RPGs): while traveling across the Point-and-Click Map, you could be interrupted by encounters ranging from a peaceful traveling dwarf merchant who had nice items to sell, through regular enemy ambush, to unique (i.e. not revisitable) levels that were part of a party member's character arc (and only triggered if he or she was in the active party). Furthermore, the regular enemy encounters varied depending on your progress in the main quest: for instance, you could encounter Darkspawn and receive help from a group of mages or knights if you have helped them earlier. All such instances were hand-built but they were still triggered randomly, so you couldn't anticipate which one you will blunder into next. Moreover, some of them (particularly in Denerim) were considerably harder than enemies in static levels, and the fact that you were transported directly to your original destination after them without a chance to resupply meant that you could run into a second tough battle straight away. Also, since even the character arc-relevant encounters were random, it was possible to go through the entire game (or, particularly, the expansion) and never get a chance to finish a companion's personal assignment.
Dragon Age II dropped the random encounters on the global map completely (except for a single mandatory encounter late in Act II), yet introduced "random" encounters with street thugs in Kirkwall after nightfall.
Shin Megami Tensei games that have Random Encounters are pretty infamous for this - it's undeniable that, good or not, the rate is INSANELY high, to the point of physical pain. While this is "good" in terms that "adds to the games' Nintendo Hardness", playing Shin Megami Tensei I for 5 hours straight can test a man's sanity.
There are exceptions: Persona 3 and Persona 4 allow you to see and attack enemies (portrayed as blobs and floating balls called "Shadows") on the field. As they are a mix of turn-based RPG and turn-based strategy, Devil Survivor and Devil Survivor 2 also do not feature Random Encounters, as you see them on the field and the game will even notify you if picking a certain event on the Overworld will lead to a battle.
In spite of Persona 3 and Persona 4 having no random encounters, Persona and Persona 2 are full of them.
Suikoden games are fairly reasonable with the Random Encounters in general, but the fourth game has rather high encounter rate to the point of frustrating. The encounter rate in Suikoden Tierkreis is nowhere as bad as the former, but it can be annoying as well.
Infinite Space allows the player to set the encounter rate higher than usual, which really helps to farm money and Fame.
Xenogears is really bad about this in one area — for what seems like ten straight hours there is ONE monster you encounter OVER AND OVER AND OVER.
Quest64 had a great many of these, to the point of a battle every few steps.
Crisis Core has random battle hot spots - locations on the map where random battles occur. In narrow areas with defined rooms they usually trigger once per room assuming you enter and leave the area immediately. If you stay in one of these rooms, they don't stop. In locations without rooms (like outside), they can trigger as often as once every other step.
Phantasy Star IV actually subverted the "Timmy gets missed by all the dangerous monsters" bit when you take a sidequest to find a lost child. One actually got him, but you fortunately manage to beat the monster and rescue him before he gets digested.
In the Monster Hunter games, a quest may have an "Unstable" hunting ground. This means that randomly (or not, in some cases), a large monster will show up to complicate matters. Tri took the trope all the way with hunting in Moga Woods, as after dealing with the initial boss monsters mentioned in the forecast, other monsters of any available type may show up randomly.
Parasite Eve has random encounters up the ass, but the rate grows lower if Aya's levels are much higher than the strength of the enemies in the area. This can make grinding for some items a pain in the ass due to enemies popping out at a low rate.
Parodied in Turn Based Battle, where you step out of your door into your first random battle...with what turns out to be the final boss. He goes through more powerful forms while the rest of the party turns up on your side.
The Knight chapter and Final chapter of Live A Live feature this, which is odd since none of the previous chapters used it.
Etrian Odyssey has a strange variation of this. On the bottom right corner of the screen, there's a circle that changes colour depending on how close you are to encountering an enemy. This can be reset via switching floors and going back on the first game, but only gives you a few free steps in the third. It's even entirely possible for the player to take a few win a battle, take a few steps, then enter another battle four steps later! Alternatively, it's possible to go through an entire floor without encountering a single enemy. Fortunately, there are skills that decrease (or increase) the encounter rate. The Farmers in 3 even have a skill that temporarily removes random encounters entirely!
Digimon World 3's encounter rate doesn't have any balance at all, to the point where you may be able to run across an entire sector without encountering a single enemy, only to encounter an enemy every two steps in the next sector.
The original Breath of Fire has a pretty ridiculous encounter rate, even when held against other similar games. The developers must have realized this, since they included a merchant in the very first town that sells Monster-repelling marbles. Smart players should stock up immediately for the sake of their sanity.
The Nintendo Game Boy game Gargoyle's Quest had a small but interesting twist - it was a typical role-playing game, but walking around in the world during overhead view had a chance to throw you into an action/platforming battle sequence with some mooks. Significant because it was more engaging than the typical turn-based battles, and not many RPGs at the time had a system like this. Being a spin-off of Ghosts and Goblins, the platformer levels played in the same way as those games.
The overworld in Neverwinter Nights 2: Storm of Zehir is full of these, with the encounters ranging from bog-standard kill-all-the-monsters to found-some-loose-change or the equivalent. There's even a number of scripted encounters on the map, some featuring characters from the previous two campaigns.
Byteria Saga: Heroine Iysayana has random battles on the world map and in a few special dungeons. Version 1.0 of Chapter One (the game had originally been episodic) had them in all dungeons, but a new version with enemy sprites appeared some months later.
Bravely Default allows the player to set the encounter rate manually from normal, to double, or to none at all. This is especially useful when your party is running low on HP and needs to escape to the nearest inn, or if you're in a hurry and need to obtain a random drop for a quest.
Shoot Em Up
The Star Control series uses these, liberally interspersed with predefined encounters for plot-relevant events.
In Star Control 2 there are generally only two kinds of ships you can encounter in most areas of Hyperspace ( the native race and Slylandro Probes), sometimes more when two territories overlap. You generally know who you're about to meet.
Slave Maker has random encounters whenever your slave went for a walk. There are some determining factors, such as stats and time of day, but for the most part, who you encounter and what happens is pretty random.
The Wing Commander series played with this now and then, particularly in Wing Commander Privateer, and in the FMV-based games.
The Disgaea series has these in the form of the various pirate crews that show up in the Item World, their strength fluctuating between being the same as the other enemies on the floor, to that of Bonus Boss levels. They generally appear within the first few turns taken on a floor, and initially are rather rare, though after defeating a group, you get access to something that can be used to make them more common. Defeating them is a requirement for getting access to the toughest post-story content, which can be a pain, as you need to not only hope they show up in the first place, but hope that it's the right pirate crew.
Makai Kingdom has a unique way of pulling these off: Each stage has a number of expansions that are triggered when you destroy an item or character with a stage "key", or when something is thrown or invited onto the new area's space. In random dungeons and some stages, this is a random selection of enemies and items. In addition, there's the chance that the new expansion will trigger an event that changes the enemies featured (such as a group of vampires or a Drill Tank), imposes a status effect on every character on the stage, or both, such as the "I've got NO Motivation" event, which fills the new area with a bunch of female enemies carrying cakes instead of weapons, but also hits everybody with a status effect that keeps them from gaining experience.
Sword of the Stars has the Unknown Menaces. Some, such as Von Neumans, Silicoids, and System Killers are persistent once randomly generated and will attack multiple systems until destroyed.
Turn Based Tactics
Silent Storm has these on the map in real-time. The frequency and types of encounters are dependent on the current region. Some appear for up to a minute, while others show up for only a few seconds. Two of the rarer kind of encounters are of note. One pits you against an enemy squad, commanded by a Japanese officer (in Western Europe!). Killing him nets you his shurikens and katana. Another encounter involves a UFO, surrounded by THO troops in Panzerkleins. Additionally, an energy rifle can be found near the craft that is the über version of the single-shot energy weapon carried by some THO troops, as it has full auto and a 50-shot power cell. That cell can then be taken back to the base and replicated for use by the said rifle, as well as energy cannon Panzerkleins. The energy rifle is an obvious Shout-Out to X-Com.
Also parodied in the Paranoia adventure Orcbusters, in which there was a literal Wandering Monsters Table - the monsters sat at it playing poker to pass the time until it was their turn to wander.
As the introduction already hints at, originally wandering monsters were a means to keep the player characters moving through the dungeon. Early D&D was almost more a "heist" game; the goal was to liberate as much treasure as you could from the dungeon (because that's where the real XP came from), not fight every single monster it might contain (which yielded comparatively little if any XP but at potentially much greater risk and cost to the player characters). Wandering monsters rarely had significant treasure, so the incentive was to avoid them, and the best way to do that was to not linger overlong and avoid drawing attention to the party's presence. As the paradigm of role-playing games shifted away from the Dungeon Crawl, the original reason behind random encounters became increasingly lost, but many games and scenarios nonetheless kept the practice alive simply out of habit.
Then there's Munchkin, in which all encounters are random, and you can send in "random" wandering monsters using the, um, Wandering Monster card. Usually, you do this to shaft someone, because that's why you do almost everything in Munchkin.