Esbern, {Gesta Danorum}

Meanwhile Ubbe was led by his grandfather, Esbern, to conceive an unholy desire for the throne; and, casting away all thought of the reverence due to his father, he claimed the emblem of royalty for his own head.

When Ragnar heard of his arrogance from Kelther and Thorkill, the earls of Sweden, he made a hasty voyage towards Gothland. Esbern, finding that these men were attached with a singular loyalty to the side of Ragnar, tried to bribe them to desert the king. But they did not swerve from their purpose, and replied that their will depended on that of Biorn, declaring that not a single Swede would dare to do what went against his pleasure. Esbern speedily made an attempt on Biorn himself, addressing him most courteously through his envoys. Biorn said that he would never lean more to treachery than to good faith, and judged that it would be a most abominable thing to prefer the favour of an infamous brother to the love of a most righteous father. The envoys themselves he punished with hanging, because they counselled him to so grievous a crime. The Swedes, moreover, slew the rest of the train of the envoys in the same way, as a punishment for their mischievous advice. So Esbern, thinking that his secret and stealthy manoeuvres did not succeed fast enough, mustered his forces openly, and went publicly forth to war. But Iwar, the governor of Jutland, seeing no righteousness on either side of the impious conflict, avoided all unholy war by voluntary exile.