Random Event

""Something has happened!""

- Neopets

In order to keep players from being complacent, matches from becoming repetitive, and to better simulate the vagaries of fortune, some video games include Random Events. The chaotic relative of the Scripted Event, Random Events are things that can happen, but where, when, or if they will happen are determined purely by chance. In video game parlance they're called "procs" (short for "special procedures", originally referring to the chunks of code that ran in MUDs when these events occurred), especially when they're attributes of an item that activate randomly when the item is used, but Random Events can also pop up in board games in the form of "chance" decks and the like.

Players' reactions to these usually depend on whether the events in question are beneficial or not.

See Random Encounters for an RPG subtype. If the event appears random, but is actually triggered by an action that might not be immediately obvious, see Guide Dang It.

Video Games

 * In Galactic Civilizations II, you occasionally get a random event when settling a world, typically ethical dilemmas such as "do I clear out the indigenous life-forms to make room for my colonists, or limit my people's living space?" Expansion packs introduced "mega" events that can drastically alter the state of your game. These include a sudden surge in piracy, a strange energy wave that boosts population growth across the galaxy, or the assassination of a rival faction's leader by one of your citizens, sparking a war. One of the worst is when some evil syndicate suddenly seizes power on a large clump of worlds, ignoring faction boundaries - a good way to lose half your empire through no fault of your own.
 * There are also two extra powerful mega events, the Telenath crystal, in which a random evil race gets a constantly increasing bonus to everything related to economy and production, and the Dread Lords arrive, in which a race with magical superpowers colonizes a world and begins building ships stronger than are physically possible to build for other players. They then proceed to conquer worlds while facing hundreds-to-one odds using force lighting.
 * As an example, the Dread Lords fighter, their smallest and weakest dedicated combat ship, packs a firepower of around 170. Up until they arrive, a firepower rating of around thirty is considered an average for a battleship...
 * Civilization IV: Beyond the Sword has things like wildfires that clear a forest square, slave revolts that leave cities in turmoil, the domestication of prairie dogs as novelty pets, or arranged marriages between nations' royal families and the responses of either side. Occasionally you'll also be offered a "quest," usually along the lines of "build x number of y and choose a reward."
 * Encountering a shiny Pokémon, at least from Generation 2 onward.
 * The latest iterations of Paradox Interactive games have a whole bunch of really complicated random and semi-random events (that is, events with triggers that makes an event more or less likely to trigger, but it is still random). Older games simply mixed random and Scripted Events.
 * Natural Disasters in SimCity. Assuming you didn't give in to Video Game Cruelty Potential and just mash the button to send them, of course.
 * In The Sims and its progeny, burglars, aliens, at-work events, etc. all fall under this category.
 * Many MMORPGs have this as a way to counter people just leaving the computer to do something for them. Because of the chance of a random event making something bad happen you're forced to sit and watch your character to make sure you're there for a random event.
 * Rift runs on this trope. It is extremely unlikely (almost impossible) that you'll be able to play for one continuous hour without running into a dimensional invasion, rift, or minor region event. With the first update, the major world event means even more chaos.
 * The RTS Star Wars: Empire at War had random events in the Galactic Conquest mode. The expansion pack removed them.
 * Sid Meier's Alpha Centauri had these, although you could turn them off. They are stuff like heat waves (extra energy at the base) or an industrial collapse (fewer minerals at the base).
 * Dokapon Kingdom has these on Yellow Spaces. Usually, they have monsters on them, but they have random people on them as well. Some of them are good, like Kira the wandering merchant and Mulch, but others of them border on the outright sadistic on the part of the game (such as Weber, who gives you the obnoxious "cursed" items).
 * In Spore, biodisasters and pirate attacks. When you're halfway to the Galactic Core and don't have Return Ticket, of course.
 * In Oregon Trail random events were used to simulate what could happen on the trail to Oregon back in the 19th century. These events included bandits, finding empty wagons, fire, snakebites, or disease, including, of course, dysentery.
 * Other games by MECC included a lot of random events too. Amazon Trail would give the player more direct control but could still have the player get sick from a disease that was commonplace in the Amazon and the stuff the boat could hit were random. The Yukon Trail would have the player or their partner fall (and possibly get hurt, slowing them down), get stuck in a blizzard, get stuck behind a mule train, and randomly losing food.
 * RuneScape was known for this trope to not only give players an occasional reward such as a party hat or a small exclusive item they can find anywhere but also to punish bots. The most common examples of bot punishers were when a high-level monster would appear and attack the player who was "Farming" items from a gathering skill. The level was variable, and would often be scaled to be a large threat to the attacked player. They only happened on some skills, because others like cooking would require complete player involvement, as opposed to Fishing and Mining that either happened until an item was obtained or the player's inventory was full. (River trolls were perhaps the most common)
 * King's Quest VII: While in the kingdom of Ooga Booga, the dreaded Boogieman will show up out of nowhere and if the player doesn't escape, will kill the player.
 * A potentially Nightmare Fuel example is on the Jump Start Adventures 4th Grade: Haunted Island game. When wandering around the haunted island, a ghost may block the player's path and force the player to answer a random question... and if they get it wrong, they lose health points. This is a completely random event, and it even happens in the Labyrinth.
 * Colonization is a borderline example. The circles that mark special events seem to have their nature determined randomly at the start of every game, so Save Scumming can protect you from events like "You were never heard from again." That said, what was "You discover a friendly village" in one playthrough seems to become "You discover the fountain of youth" in the next. This is because the sequence is fixed, so whether you try ruins A then B or vice versa, you get e.g. "friendly tribe" in first attempt and "fountain of youth" in second.
 * Master of Orion II: oh, yes. Aside of random proposals from heroes, there are: research advance or wipe-out, findings with tech or resources, diplomatic marriage, assassination, monetary offering, total hyperspace block, ecological calamity, surprise mineral deposit, reproductive boom, incoming comet, plague, nova, Space Pirates, monsters, Antarans... All? This can be turned off, though.
 * Master of Magic: non-linearity being one of best features of the game, it's no surprise random events may have a great impact. Global conditions that affect power income or population. And wandering monsters, of course.
 * There are, however, true Random Events (start at turn 50, with chance depending only on Difficulty level, maximum 0.2%/turn), some of which are global, other affect a Wizard or Town, some are instant, others with duration. And there are Fame events (the chance depends on Fame, all let the Wizard acquire an asset for gold) - hero (you also need enough of Fame for that Hero's tier), Mercenary (hire randomly rolled units, though they never have Magic Weapons) and Merchant (buy a randomly rolled magical item) offers.
 * The hallucinations in Trilby's Notes. Chances are you will run into at least one or two over the course of the game, but which ones and where you are when they happen are random: the game is coded so that every time you take a pill, there is a chance that a random hallucination will trigger two screens later. Granted, it is possible to take advantage of that fact by trying to trigger them on purpose, but chances are it'll take several pills and a lot of patience to trigger them all.
 * Left 4 Dead is made of this for the Tank and Witch. These two powerful special infected CAN show up at any time, but where and when is left up to the AI Director, assuming if it's not in a sadistic mood.
 * And by the way, just because you never see a Tank and Witch together, doesn't mean they can't show up together.
 * The game worlds of Dwarf Fortress are procedurally generated, and in play the kind of creatures that appear are semi-random, whether raids or seiges strike are random, dwarven moods and the products created from them are random. There are very few set events—ie same type of merchants arrive in the same season around the same date—and even those are being replaced with random distribution based on generated civilization and the fortress' own trading history.

Other games

 * In the board game Mandate of Heaven each player during their turn must draw a yin card and a yang card. One type hits you with bad events, the other either confers an advantage (like being able to pass through the Great Wall even if its guarded), or a counter to a specific bad event.
 * Monopoly has the Chance and Community Chest Cards.
 * Even for a tabletop roleplaying game, Maid the RPG takes this unusually far; there are rules giving players the explicit ability to call for a random event, and said random events can quite easily completely derail the game's current plot.