Mercia was conquered by the Vikings, and its king driven into exile, in 873/4, after which it ceased to exist as an independent kingdom. Mercian territory to the north and east of the line separating the Danelaw became part of the Danelaw, and the south-western territories became dependent upon Wessex until being completely absorbed in 918. Within a decade, the greatly enlarged kingdom of Wessex had conquered parts of the Danelaw and it is from this point onwards - 927 - that historians traditionally date the effective creation of the kingdom of England.
In consequence of its loss of independence in that late ninth century, specifically Mercian characteristics are difficult to detect in Domesday Book, being overlaid by those of their Viking or West Saxon successors. The word itself is not recorded in Domesday.
For more detail, see Ian W. Walker, Mercia and the making of England (2000); and Stephen R. Bassett, The origins of the English kingdoms (1990).