Unlockable Content earned by playing with someone who has already unlocked it
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, some people (usually staff members) usually have the item in question unlocked from the start so they can spread it to normal players. A type of Socialization Bonus, and possibly a Luck-Based Mission.
- The Creeper Clunk skin from Awesomenauts is unlocked by killing a player who is playing as Clunk and is wearing the skin.
- 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).
- 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 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 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.
- 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.
- XCOM: Enemy Unknown has an achievement named "Bubonic", which can only be earned by playing online against someone who already has it.
- In Super Smash Bros.: Brawl, playing online against a secret character unlocks that character instantly.
- The concept of an Erdos number. Mathematician Paul Erdos had a number of 0. Anybody who has worked directly with him is given the number 1. Working with somebody who has an Erdos number of 1 grants one the Erdos number 2, and so on. In a similar vein, the concept of Bacon Numbers calculates the distance between an actor and Kevin Bacon,
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