Ceirdwyn was born a Celt in Britannia in AD 29. She became a warrior in her tribe and fought against the Romans. In AD 60, she was killed while fighting the Romans in a small skirmish, and revived as an Immortal. She soon met her first teacher, Roman General Marcus Constantine, who taught her about her immortality.

Marcus convinced her to come with him to Rome with him. The two lived together until AD 73, when Ceirdwyn decided to follow her own path. She later returned to her native Britain and met her first pupil, Alex Raven, in AD 75.

Over the years Ceirdwyn has taken in many immortals as her Pupils. She taught Matthew McCormick, Ailell of Kells, and Ximena.