Ęthelswith (c. 838-888) was the only known daughter of King Ęthelwulf of Wessex. She became Queen Ęthelswith when she married King Burgred of Mercia in 853. The couple had no issue. Her marriage probably signalled the subordination of Burgred to his father-in-law and the Saxon kingdom at a time when both Wessex and Mercia were suffering Danish (Viking) raids. Burgred also had ongoing problems with the Kingdom of Powys on his western border and in 853 Ęthelwulf subjugated the Welsh state on Burgred's behalf.
Repeated Danish incursions over the years gradually weakened Mercia militarily and in 868 Burgred was forced to call upon Ęthelswith's brother King Ęthelred of Wessex to assist him in confronting an entrenched Danish army at Nottingham. This was the last time the Saxons came to the aid of the Mercians and is also notable as the occasion on which Alfred the Great, another brother of Ęthelswith's, married his Mercian wife Ealhswith.
Burgred's reign lasted till 874 when the Vikings drove him from the kingdom and he fled to Rome with Ęthelswith. He died shortly after. Ęthelswith lived on in Italy, to be buried at Pavia in 888 (wikipedia.com)