Question: Which 991 CE battle beside River Blackwater is the inspiration for an Old English poem of the same name?
Sentence (from "The Homecoming of Beorhtnoth Beorhthelm's Son"): The Homecoming of Beorhtnoth Beorhthelm's Son is the title of a work by J. R. R. Tolkien that was originally published in 1953 in volume 6 of the scholarly journal "Essays and Studies by Members of the English Association", and later republished in 1966 in "The Tolkien Reader".
Relevant evidence: No