Which of these massively successful games was originally released as a mod (modification) of another game before becoming its own standalone product?

The gaming industry is known for its spectacular product launches that can make or break a title's success. Some of the most profitable games in history had unusual or humble beginnings that defied expectations. Test your knowledge about these legendary gaming product launches that transformed the industry landscape and created new gaming paradigms. Do you know which unexpected hit revolutionized its genre?

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Counter-Strike (originally a Half-Life mod)
World of Warcraft (originally a Warcraft III mod)
Fortnite (originally an Unreal Tournament mod)
Minecraft (originally a Roblox mod)