Prospero began in late 1996, some time after Valve was founded. The game was to emphasize exploration, an intricate storyline, and combat with "psionic" powers. The game was Influenced by the video game Myst, and the works of Jorge Luis Borges. The protagonist of Prospero (known as "The Librarian", "Aleph", or "Aleph-Null") underwent a series of design changes during the early development of the game. One iteration relied heavily on the use of psionic amplifiers to augment her innate abilities.
As the design of Quiver (the working title for Half-Life) started to take over some of Prospero initial goals, Prospero evolved into a massively multiplayer game. It was also intended to be distributed with a mix of official and user-created worlds that could be accessed through "The Library," a core hub of the game, where the main character was in charge of the various world stored there. Each game would be running on its own server. Online distribution, server browser, a friend finder, user-created content and other concepts initially conceived for Prospero would eventually find their way into the Half-Life series, the Portal series, the Steam online service, and Valve's support of fan-made modifications. Unfortunately, Half-Life's development proved an irresistible force to the development team, and Prospero's development would ultimately be abandoned.
In 2014, screenshots of the game (showing rough and unfinished environments) were finally revealed, and were presented by ValveTime.
- In the Half-Life sound files folder, the music files "Dimensionless Deepness", "Steam in the Pipes", "Threatening (Short)", "Traveling Through Limbo" and "Vague Voices" are named "prospero01" to "prospero05."
- In the Half-Life 2 Beta sound files, Wallace Breen can be heard mentioning the "Aleph universe" as an early name for the Combine Overworld.