A Viral Unlockable is a piece of Unlockable Content such as an achievement, skin, or weapon, that is earned by playing with another player who has already unlocked it. Depending on the game, merely meeting such a player may be enough, or you might have to defeat (or be defeated by) the player in question.
Because of the obvious Catch-22 Dilemma involved with this trope, someone has to get it some other way so they can spread it to normal players. Methods includes time-limited events, an alternate (often much more difficult) unlocking method, or some people (usually staff members) being given it immediately.
Card Battle Game
- Hearthstone: Heroes of Warcraft, as a reference to the Corrupted Blood incident, made the Hakkar card back unlockable for most players by playing against anyone else who has the Hakkar card back. It also automatically switches your card back to that, so you could be unintentionally spreading it if you didn't check.
- In Super Smash Bros. Brawl, playing online against a secret character unlocks that character instantly.
First Person Shooter
- Borderlands has "And They'll Tell Two Friends", a trophy/achievement unlocked by playing co-op with either a Gearbox Software employee or another player who's earned it.
- Some title cards in the Call of Duty series are unlocked in this manner. For example, the "STD" and "Infected" titles in Call of Duty: Modern Warfare 2 and 3 are earned by using your knife to kill someone who has the title equipped.
- The first Left 4 Dead has the "Outbreak" achievement that is passed on by Boomer players puking on people without the achievement.
- PAYDAY 2 has the mysterious "Virus" achievement. Its only description is you need to play with someone who owns that achievement to get it.
- Team Fortress 2:
- The Pyrovision goggles can be earned by dominating a player who was wearing them. Seeding was done by just giving a pair to anyone who launched the game in a limited time.
- The infamous Ghastly Gibus. Again earned by dominating someone wearing the hat. Seeded by a semi secret webpage.
Multiplayer Online Battle Arena
- The Creeper Clunk skin from Awesomenauts is unlocked by killing a player who is playing as Clunk and is wearing the skin.
- In Heroes of the Storm, Cho'gall was initially released in this way - a two-headed character controlled by two players at once, you could play as one of the heads as long as someone who had unlocked him was playing the other. After doing this, he was unlocked for your own use.
- The Xbox 360 version of Need for Speed: Carbon has multiple achievements that could be obtained by completing certain online gamemodes with other players that already have the achievement. Each achievement is named after their gamemode (for example: Knockout Fever for Pursuit Knockout races), and if you obtained all three viral achievements, you would unlock the Carbon Plague achievement, granting you a unique black virus-themed livery for your car.
- Brütal Legend has the "Six Degrees of Schafer" achievement, which can only be obtained by playing an online game against someone who already has it (with the original vector being, of course, Tim Schafer).
- The CROSS×BEATS REV. song "Blue Destiny Blue Eternal" can only be unlocked by playing a multiplayer session with someone who has the song unlocked, and then playing the song in said session.
- Several jubeat games allow you to unlock songs simply by playing them in local or online multiplayer rather than through their "proper" unlock conditions. In the case of online, you pick a "matching select" option instead of a specific song and the game will pick a song that others on the network are currently waiting to play; if the song is not one you've unlocked yet, you will obtain it. In fact, one song could be unlocked by online-matching with particular members of Konami staff, although the usual "pick matching select" and "have someone else pick it in local matching" methods work too.
- The post-game DLC quests for Dragon Quest IX have become this since the DS' Wifi Connection servers were disabled- the quests can still be unlocked via local wireless connection with a player who already has them, making them one of the few bits of DS DLC that can still be legitimately obtained post-shutdown.
- In Pokémon Omega Ruby and Alpha Sapphire, the Eon Ticket item (allowing the owner to battle Latias or Latios depending on version) was distributed via StreetPass, meaning that getting it required you to be near someone else who had the Ticket and who had their 3DS with them. This made getting it fairly easy in populated areas, but very difficult for players who were more isolated.
- XCOM: Enemy Unknown has an achievement named "Bubonic", which can only be earned by playing online against someone who already has it.
- In the Splatoon games, it's possible to order pieces of clothing gear from the other players in your plaza. Upon placing an order, you'll be able to purchase a copy of that gear item from Spyke or Murch the following day. While this system does have its limitations — some items that are obtained as part of promotional events or unlocked in the single-player campaigns cannot be ordered — several gear items that would otherwise be unobtainable Temporary Online Content can still be acquired through this method.
- In Dragon Cave:
- The Tinsel and Shimmer-scale breeds are only available as contest prizes or having it bred. For a long time all Tinsels and Shimmer-scales on the site were descended from the 5 gold/10 silver/15 bronze Tinsels and 10 gold/20 silver/30 bronze Shimmer-scales given to the original winners of their respective contests, but they have since been made much more obtainable (e.g. monthly raffle).
- For Valentine's Day, Halloween, and Christmas, there's a new limited-time release each year that drops only for 24-72 hours depending on the event. The only way to obtain these breeds in subsequent years is to get one that was bred by another user.
- The concept of an Erdős number. Mathematician Paul Erdős had a number of 0. Anybody who has worked directly with him is given the number 1. Working with somebody who has an Erdős number of 1 grants one the Erdős number 2, and so on. In a similar vein, the concept of Bacon Numbers calculates the distance between an actor and Kevin Bacon.