The elderly librarian of Sarpeidon, who used the atavachron to send his entire civilisation into the planet's past before its sun, Beta Niobe, went nova, and who accidentally does the same to Kirk, Spock and McCoy.
Mr. Atoz appears in the 1969 episode All Our Yesterdays, played by Ian Wolfe. He is the last person left on Sarpeidon, keeping a library of discs that show periods of the planet's history, assisted by several replicas of himself.
Mistaking the Enterprise landing party for late-arriving citizens, he urges them to choose an era. Kirk steps through the atavachron portal into an age of witch trials, while Spock and McCoy end up in an ice age five thousand years earlier.
When Kirk returns and tries to recover his officers, Atoz attempts to force him back through, since he believes nobody can survive outside their chosen time. Once the three are reunited, he slips through the portal to join his wife and family moments before the nova.