It is quite certain, [..] that Ethelred king of Mercia, was a great benefactor to [Bardney Abbey]. And when this monarch relieved himself from the cares of government after a long reign of war and bloodshed, in which he had recovered "the isle of Lindsey" from the Northumbrians, and ravaged the kingdom of Kent, sparing neither age nor sex, church, nor monastery, by resigning his kingdom; to atone for his misdeeds, he retired to spend the remainder of his life in Bardney Abbey, and accepted the office of its abbot [..]. A large barrow or tumulus still remains near the site of the abbey, where tradition says he was buried. It is called to this day "King's (Conig) Garth."
p35 of 'An account of the religious houses formerly situated on the eastern side of the river Witham' by the Rev. George Oliver (1846). Online at Google Books.