Eclipse was a failed test of the original Daycare Attendant animatronic, when his wiring that allows him to switch between Sun and Moon got crossed and managed to make him both angry and vengeful, the tech crew locked him away in the Daycare storage facility. But when Freddy goes in to talk to the miscreation, Eclipse rises and makes Freddy's heroism experience a living hell.
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Eclipse's voice should sound similar to Moon's voice from Security Breach, but much more sinister with a deep growl trailing behind after every sentence.