Mercia

Mercia is the name of a kingdom in Albion that is ruled by Bayard.

History
The kingdom was at war with Camelot for many years. Finally, Bayard and Uther Pendragon agreed to a peace treaty, which was almost upset when Nimueh framed Bayard for the attempted assassination of Arthur Pendragon. Uther's court physician Gaius uncovered the truth before Mercia's troops attacked Camelot for Bayard's imprisonment, and the two kingdom's presumably reached a peaceful settlement (The Poisoned Chalice).

When Morgana and Gwen were kidnapped and Kendrick's men intended to hold them hostage, Arthur believed Mercia was behind it because of the swordcraft (Lancelot and Guinevere).

Lord Bayard is later called a "friend to Camelot" by Uther (Beauty and the Beast: Part Two).

When Arthur went on a quest for the Fisher King's triton, Mercia appeared on the map so it's likely that he went past the kingdom (The Eye of the Phoenix).

When Arthur became King of Camelot, Agravaine questioned that he is a threat and is not to be trusted, despite the peace treaty (His Father's Son).

In The Legend
In actual history, the kingdom of Mercia was Anglo-Saxon, established after the supposed death of Arthur. It covered much of south Derbyshire, Leicestershire, Nottinghamshire, Staffordshire and Northern Warwickshire.