For anyone curious about legal history I offer this as a footnote to two articles on this site, The mutual wills problem and Legal style through the ages, the second of which has an example of an Anglo-Saxon will.
According to Justin Pollard's book Alfred the Great: The Man Who Made England [the essence at p.126 but on various other pages]:
In 871 CE Viking bandits and settlers already controlled most of England except Wessex, the area along the south coast (except Cornwall) approximately as far north as the Thames Estuary and the Bristol Channel.Although Pollard doesn't say whether Æthelred's will contained a similar clause, the agreement was apparently implemented as, when Æthelred died (of an unknown cause — possibly wounds) 4 weeks after the battle, Arthur succeeded peacefully to the throne, on which he remained for 28 years until his death. Again according to Pollard, he radically altered the strategy and method of defence, reorganised the administration of the kingdom, beat off the Vikings, and began the translation of legal, religious, and other important documents from Latin into English so that they would be accessible to as many people as possible. Other parts of the country accepted his leadership, beginning the unification of England that was completed by his descendants. The remaining Viking settlers were assimilated.
Now they were trying to conquer Wessex, and Æthelred (the Æ pronounced as in "cat"), King of Wessex, and his younger brother Alfred, were about to go into battle against them.
Under Saxon law, if Alfred survived his brother he (and eventually his children) would inherit the throne but not necessarily the resources to rule it. This might have led to a civil war aimed at putting Æthelred's children on the throne but the effect of which would be to lose Wessex to the Vikings.
So Æthelred and Alfred agreed (as recorded in Alfred's will) "that whichever of us live longer should succeed both to lands and treasures and to all the other's possessions except the part which each of us had bequeathed to his children" [quoted in modern English translation from English Historical Documents, vol 1, edited by Dorothy Whitelock].