Aelfthryth was the widow of Edgar's foster brother and the marriage was controversial. The relationship was close enough to cause shock waves and there may have been a matter of suspected adultery and murder. Twelfth century accounts claim that Edgar and Aelfthryth were so enamoured with each other that they actually plotted the death of Edgar's foster brother so they would be free to marry.
Aelfthryth allegedly plotted the murder of her young stepson King Edward The Martyr so that her son Aethelred could claim the throne. She was an extremely forceful figure at court int he early years of Aethelred's reign.