The Calculator is the main antagonist of Fallout Tactics. It is a supercomputer that was created as part of Vault-Tec's Enclave-manipulated social experiments and was made to act as the Overseer for Vault 0, where it had power over managing the vault's food supply and other resources for the cryogenically-frozen population.
The Calculator is not truly a robot but actually a cyborg and multi-being, as its CPU is collectively made up of the cryogenically frozen brains of eight of Vault 0's most talented original residents: A scientist, a politician, a doctor, an artist, a lawyer, an Evangelist, a video game designer and a porn star.
Not too long after its installation, the Calculator went insane and ordered the robots it controlled to kill the people of the vault. Those who were not killed by the robots suffered severe brain damage due to the Calculator coming to neglect its duties in sustaining the life-support systems. Many years later after this incident, a group of Super Mutants stumbled into Vault 0, which activated the Calculator's security protocols which, for its current corrupted state, prompted the machine to declare total war on all organic life and it began building robot factories to add to its robotic forces for an invasion of the wastes to the East.
Eventually, the Midwestern portion of the Brotherhood of Steel was at war against the Calculator's machines, and their general Simon Barnaky was captured by the Calculator and transformed into its personal right-hand robobrain. At the climax of the game, a BOS squad led by the "Warrior" (the player character) enters Vault 0, makes it past all the defenses including Barnaky, and destroys the Calculator's brains. The brains were the part of the Calculator that had gone mad, not the machinery itself, so the player gets to make the final decision of what to do with the Calculator. They may let the machine shut down or destroy it outright, or give it a new mind with either the Warrior's brain, or Simon Barnaky's, if he had not been killed in the previous fight. If the Warrior becomes the new Calculator, it outlaws discrimination against mutants but may or may not be a corrupt ruler depending on the Warrior's established level of morality, while if Barnaky's brain is installed it becomes outright violent and genocidal towards mutants, driving them all the way to the West coast where it is implied they will be wiped out by the similarly xenophobic Western Brotherhood of Steel.