One of the first things Uhtred learns when he's taken as a slave by the Danes is don't mess with Ubba. Pretty sound advice if you pay attention to the way this nut goes about his business.
Even as Uhtred's fighting skills advanced, he was pretty nervous fighting Ubba. It's one of the few
times you see Uhtred rattled with a sword in his hand.
But as he always does, Uhtred took down the Dane leader, sending a ripple across the Dane invasion that has allowed countless, inferior fighters and leaders to take their shot at Uhtred.
Had Ubba won, the Danes would have gained complete control of England.
Also, this was a pretty gross death. Uhtred slices his Achilles to get the upper hand. A visual that'll send a shiver down your spine.