Which game engine was the first to offer a subscription model where developers could access all features for free, only paying royalties after their game reached significant commercial success?

The world of game development has evolved dramatically over the decades, with certain products and platforms revolutionizing how games are created. Test your knowledge about the tools and technologies that have shaped modern game development and virtual worlds. Can you identify the breakthrough product that changed how developers approach game creation?

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Unity Engine
Unreal Engine 4
CryEngine
GameMaker Studio