Aleph One is an open-source game engine that allows users to play and modify the classic first-person shooter game Marathon, originally developed by Bungie. It supports various platforms, including Windows, macOS, and Linux, making it accessible to a wide range of players. The engine enhances the original game experience by providing improved graphics, customizable controls, and support for user-created content. Players can create new levels, scenarios, and even entirely new games using the tools provided by Aleph One, fostering a vibrant community of developers and fans dedicated to the Marathon series.