As far as I remember he killed his enemy, tied his head onto his saddle, at which point the enemy's tooth cut his leg and the king/leader died a while later from the infection. I have a suspicion this is a character in a medieval Norse saga, or an Anglo-Saxon king, but I can't remember who. Thanks!
This story appears in the Orkneyinga saga, describing how Sigurd Eysteinsson (aka Sigurd the Mighty, ruled circa 875–892) defeated an indigenous Orkney ruler, Máel Brigte the Bucktoothed, whom he beheaded. Having then tied the head to his saddle, Sigurd received a scratch from the tooth of his dead opponent, and the wound became affected, etc.