This is Layamon's Brut, written about 1205 in an alliterative prosody that is clearly directly descended from the Old English poetic measure, though the subtleties of the older measure have disappeared. Apparently, at least for oral purposes, alliterative poetry never ceased to be composed, as is further suggested by its reappearance later in written form in the so-called "alliterative revival" of the 14th century, which culminated in Piers Plowman and the Green Knights.
Source: English Literature / Middle English Literature
I know the meanings of each word written in blue but I can not understand the blue sentence as a whole.
Would you please clarify it in a simple form so that I can understand it better?