From the Medieval Research Blog: "Aþum Swerian: Swearers of Oaths?"

Beowulf is a story about a doomed people who are destined for annihilation as a result of depredation, feuding, and cyclical inter-tribal violence. Yet, the violence described in the poem is not always outward but often occurs from within, as acts of intra-tribal violence frame much of the narrative. Even seemingly positive events are thus generally short-lived. Accordingly, in the eminence of King Hrothgar’s glorious construction of Heort, the narrator reveals the hall’s imminent doom:

Sele hlifade
heah ond horn-geap. Heaðo-wylma bad
laðan liges. Ne wæs hit lenge þa gen
þæt se ecg-hete aþum swerian
æfter wæl-niðe wæcnan scolde. (81-5)

The hall sheared upward, high and horn-vaulted. For the battle-surge it waited, loathsome fire. Nor was it long before the edge hate of aþum swerian must awaken for slaughter-spite.

This dire prediction identifies the causal agents of disaster as aþum-swerian. But given that this term is unattested and grammatically invalid, we are bound to ask: Who are these aþum-swerian? The conventional approach solves this conundrum by creating a new term in imitation of such copulatives as suhtergefaedaran (“nephew and uncle” from Beowulf), gisunfader (“son and father” from Heliand), and sunufatarungo (“son and father” from Hildebrandslied). Following these models, the editors of Klaeber 4 (120, 350, 437) emend the term to aþum-sweoran, thereby conjoining aþum (sons-in-law) and sweoran (fathers-in-law). Because this solution apparently predicts the sundering of vows between Ingeld and Hrothgar (2022-66), this emendation has become the dominant convention. 

Nevertheless, there are problems. First, the emended term, glossed as “sons-in-law and fathers-in-law,” differs markedly from the models, which are glossed as “nephew and uncle” and “son and father.” And though the term aþis indeed attested with the gloss “son-in-law,” the rendering aþum-sweoran is a hapax legomenon attested nowhere in the extant corpus of Old English literature. Making the invention yet more suspect is the well-attested phrase, sweor ond aþum (father-in-law and son-in-law), which would seem to preclude a need for the copulative. 

This is an excerpt from "Aþum Swerian: Swearers of Oaths?" by Drs. Chris Vinsonhaler & Richard Fahey. Read the full story.