June 12, 918. Æthelflæd, Lady of the Mercians, dies at Tamworth
Edward occupies Tamworth, receives submissions of all the English Mercians (and some Welsh kings)
December 918. Ælfwyn, Æthelflæd's daughter, taken into Wessex
On June 12, 918, Æthelflæd died at Tamworth, seven years after the death of her husband Æthelred in 911. The Mercian annals note that this was in the eighth year in which with lawful authority she was holding dominion over the Mercians, and that she was buried at Gloucester. They continue in their next annal that three weeks before Christmas, Ælfwyn, daughter of Æthelred, lord of the Mercians, was deprived of all authority in Mercia and taken into Wessex.
The main body of the Anglo-Saxon Chronicle (or what can more properly be called the West Saxon version) states only that Edward's sister Æthelflæd died and that Edward occupied Tamworth and all the Mercians who had been subject to Æthelflæd submitted to him, as did also three Welsh kings, Hywel, Clydog and Idwal. As far as West Saxon observers were concerned, Æthelflæd was to be the last separate ruler of Mercia.
It is clear from several sources that Edward was in overall charge of both Wessex and Mercia in the first twenty years of the 10th century, and the change in 918 was not so much Edward taking over Mercia as Edward removing an intermediate level of government between himself and his Mercian subjects. Edward commanded armies of West Saxons and Mercians in 909 and 910; he seems to have issued coins in his own name throughout Wessex and Mercia (no coins survive in the names of Æthelred and Æthelflæd); the charters make clear that Æthelred and Æthelflæd hold their power under Edward's authority (e.g. S 367 of 903).
However, the references to Æthelflæd's "legitimate authority" and Ælfwyn being "deprived of all authority" in the Mercian annals make it equally clear that at least one Mercian observer expected that the Mercians would keep their own governor. It is most unfortunate that no royal diplomas survive from the period, so we have no evidence of whether Ælfwyn ever exercised authority in Mercia. A charter from the second half of 918 in Ælfwyn's name, witnessed by the members of her court, might offer a fascinating glimpse into the politics of the period. (The West Saxon version of the Chronicle, which states that all the Mercians submitted to Edward shortly after Æthelflæd's death, implies that the West Saxons would see any attempt by Ælfwyn to issue charters in her own name as a revolt -- but the way the Mercian annals suggest that Ælfwyn was deprived of authority may suggest the attempt was made.)
The most tantalizing question about Mercian politics in 918 is what part was taken by Edward's son Æthelstan. Pre-Conquest sources tell us little about Æthelstan's upbringing, but the 12th-century William of Malmesbury notes that he was brought up at the court of Æthelred and Æthelflæd. If this is so, he may have played a key part in these events, either for Edward or for Ælfwyn, and his actions here might well relate to his own apparently problematic succession in 924/5.