This is a reference to the Battle on the Ice of Lake Vänern, during which Eadgils slew Onela and which also appears in the Skjöldunga saga.
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These sources also deal with his war against Onela, which he won with foreign assistance: in Beowulf he gained the throne of Sweden by defeating his uncle Onela with Geatish help, and in two Scandinavian sources (Skáldskaparmál and Skjöldunga saga), he is also helped to defeat Onela in the Battle on the Ice of Lake Vänern, but with Danish help.
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Snorri relates that Aðils was in war with a Norwegian king named Áli (Onela), and they fought in the Battle on the Ice of Lake Vänern.