According to urban legend, Polybius was a strange arcade game, similair to Tempest which brainwashed those who played it and was placed in arcades by a shady branch of the federal government. All retellings have a different agency, from the CIA, to the NSA, to the FBI, to even forign agencies like the KGB. In reality - at least, according to a former Atari employee - Polybius was a name for one of the early Betas of Tempest, and was tested in a few arcades. The game was known to cause Vertigo and Motion Sickness, and as such was pulled immediately.
My cousin's friend's older brother played Polybius and went insane. I swear!