One of the most powerful Anglo-Saxon kingdoms based in the English Midlands.
This is where I live. Technically it's Staffordshire but it used to be Mercia, meaning 'border people'. It's origins are a bit hazy but evidence has shown that the Angles inhabited the area around the River Trent by the sixth century, the first king or chieftain being Icel in about 527.
The capital of Mercia was Tamworth (previously known as Tameworth and Tomtun) and the site of Tamworth Castle is the approximate site of the first royal fortress, built in the 580s. The rise of the kingdom started with Penda, who has the dubious reputation of being the top regicide by killing five kings. He was also the last pagan king of England but had no problem with the missionaries from Lindisfarne spreading the word.
Penda's sons were also kings and Wulfhere reclaimed the territory lost when Penda was killed by the Northumbrian king, Oswiu. After that brief blip, Mercia continued to be a powerful kingdom and reached its height in the mid eighth century when Offa became king. He is famous for the wall separating Mercia from Wales - Offa's Dyke. Although now there is evidence that its formation started earlier. Offa created the first gold coins, market towns and negotiated with Charlemagne as an equal. This period between 600 and 900 was named the 'Mercian Supremacy' by the historian Sir Frank Stenton.
Mercia lost power in 918 when it was annexed by Wessex. The ruler then was Aelfwynn, the daughter of the Lady of the Mercians, Aethelflaed, who succeeded her after her mother's death. She lasted only six months, some theories say that she went into a nunnery but either way Wessex had taken Mercia under its wing. In the eleventh century Mercia was divided between Canute and Edward Ironside. And everything ended when the Normans conquered in 1066 anyway.
Mercia also has the Staffordshire Hoard, a collection of treasure found stashed in a farmer's field. From this we learned that Mercia - as well as all of the Dark Age kingdoms - was much more advanced and sophisticated than we first thought.
J.R.R. Tolkien was an expert in the Mercian dialect and I like to think that some of the landscape in Lord of the Rings was based in my town. All of my Dark Age books are set in Mercia. Therefore, every day is research day, no matter what I do.