Short answers, pulled from the story.
Mercia was an early medieval English kingdom centred on the River Trent and its tributaries, in the region now called the Midlands of England. It was one of the seven kingdoms of the Anglo-Saxon period and had no fixed capital city; the royal court moved throughout the kingdom.
Offa reigned from 757 to 796 and became the greatest king Mercia ever produced. He dominated southern England, founded market towns, oversaw the first major issues of gold coins in Britain, negotiated with Charlemagne as an equal, and is credited with constructing Offa's Dyke as the boundary between Mercia and the Welsh kingdoms.
The Staffordshire Hoard is a collection of Anglo-Saxon gold discovered in July 2009 by Terry Herbert in a field at Hammerwich, near Brownhills in Staffordshire, close to Lichfield. Scholars Svante Fischer and Jean Soulat tentatively dated the artefacts to the 7th-8th centuries, placing them in the era of Mercian power.
Æthelflæd, born around 870, was the daughter of Alfred the Great and wife of Æthelred, Lord of the Mercians. She assumed power when her husband fell ill and after his death in 911 ruled as "Lady of the Mercians". By 918 she and her brother Edward the Elder had conquered the southern Danelaw. She died on the 12th of June 918.
Tolkien based the language of Rohan on the Mercian dialect of Old English and named the kingdom the Mark, a word cognate with Mercia. Several kings of Rohan share names with actual Mercian monarchs from the royal genealogy, including Fréawine, Fréaláf, and Éomer. Tolkien scholar Tom Shippey also argued that the Middle Kingdom in Farmer Giles of Ham is based on Mercia.
Mercia's last king, Ceolwulf II, died in 879, after which the kingdom was governed by lords under West Saxon overlordship. Mercia briefly regained separate political existence in 955-959 and again in 1016, when Cnut took it as his share of a divided England. It remained an earldom until the Norman Conquest in 1066.