Mercian was a dialect spoken in the Anglo-Saxon kingdom of Mercia (roughly speaking the Midlands of England an area in which four kingdoms had been united under one monarchy). Together with Northumbrian, it was one of the two Anglian dialects. The other two dialects of Old English were Kentish and West Saxon. Each of those dialects was associated with an independent kingdom on the island. Of these, all of Northumbria and most of Mercia were overrun by the Vikings during the 9th century. Part of Mercia and all of Kent were successfully defended but were then integrated into Wessex. Because of the centralisation of power and the Viking invasions, there is little or no written evidence for the development of non-Wessex dialects after Alfred's unification, until the Middle English period.
The Mercian dialect was spoken as far east as the border of East Anglia and as far west as Offa's Dyke, bordering Wales. It was spoken as far north as Staffordshire, bordering Northumbria and Strathclyde, and as far south as South Oxfordshire/ Gloucestershire, where it bordered Wessex. The Old Norse language also filtered in on a few occasions after the foundation of the Danelaw. This describes the situation before the unification of Mercia.
The Old English Martyrology is a collection of over 230 hagiographies, probably compiled in Mercia, or by someone who wrote in the Mercian dialect of Old English, in the second half of the 9th century.