Prospero is only a few steps shy of being an urban legend.
Early in Valve's history, the developer worked on two projects: Quiver, which would later become the Half-Life series players know and love today, and the far more mysterious Prospero. It's unclear how far Prospero made it before being scrapped, but pieces of it continue to surface even today.
Fansite ValveTime released a video exploring the project's murky history. Along with alleged screenshots of the game, ValveTime includes brief insight from employees such as writer Marc Laidlaw, who has previously talked about the game in brief. On Gamasutra, the writer revealed that in 1997, he was initially devoted to "a science fantasy epic" called Prospero. The game was said to emphasize exploration and intricate combat.
Although the team was "absorbed" and the project seemed to vanish, elements of Prospero seem to have survived throughout Valve's catalogue. As pointed out in the video, former Valve game designer Karl Deckard mentions on LinkedIn that he worked on Prospero, "much of which was later repurposed for the Half-Life series and the Portal series."
Watch the video above for a look at the game's early concepts.