The Moon Cell is a fictional supercomputer from the visual novel series Fate/Extra. It is located on the Moon and serves as a vast repository of knowledge and a powerful entity that can simulate reality. The Moon Cell is capable of creating digital worlds and is central to the events of the game, where it hosts a battle royale known as the Holy Grail War. In the context of the series, the Moon Cell interacts with various characters, including Masters and Servants, who compete for its power. The outcomes of these battles can alter the fabric of reality, making the Moon Cell a crucial element in the overarching narrative of the Fate universe.