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... Agrippinae nuntium Agermum, scaenam ultro criminis parat, gladiumque, dum...

Anicetus took it upon himself to bring the accusation because Nero, in his panic, assumed Agippina's freedman had come to bring a charge agianst him. "ultro" means "on one's own initiative," "of one's own accord"; it implies doing something beyond what is asked or expected.