It is with relief I see you have kept "Grendel" out of the title. (Pardon, but I am going on the assumption this is from Beowulf). Of course, it is so artfully done, it could as well apply to humanity in a spiritual sense of "monster."
Great relation of his mother's protective physical carapace, with grief. Never did I find the repetends and rhymes cloying or co-erced, except perhaps ever-so-slightly in S5, with "reef." Stanza 4 is beautiful, with its allusion to Hrothgar's man dragged to the Mere. The lake bed bit is such a powerful one, in fact, you might strive to replace "arrives" of line three to something more germane and anchored to it -- "ascends"? Strong idea of her emerging from the depths, and a clearer pictograph, methinks, than "arrives."
Line 7 -- so touching and true. I like the duplicity in the following line, as well. "oblivion is chief" is another memorable phrase.
Not heavily introspective, and illuminating enough, psychologically, to be a satisfying piece. In form, probably no less accomplished than Bishop's "Art of Losing."