The Viking king of Jorvik, Sigtryggr, who appears in Saving the Unicorn's Horn, is not my invention, but was actually a real person. Sigtryggr was the Old Norse name for him, but he was also known as Sitric Cáech or Sihtric Cáoch. Since 'cáoch' or 'cáech' means "one-eyed" in old Irish, it's quite possible that he had lost an eye in battle.
Sigtryggr was certainly no softy: in 888, he killed his own brother. He fought many battles too, including several against the High King of Ireland, Niall Glúndub. In 919, they fought a battle in which Niall Glúndub was killed and the Irish were defeated, after which, Sigtryggr became King of Dublin. This is the battle that Joe helps to re-enact at the end of "Saving the Unicorn's Horn".
What Lucy tells Joe about Sigtryggr converting to Christianity is also true. In 926, Sigtryggr married the sister of the Anglo-Saxon king Æthelstan, and agreed to follow his new wife's religion. But a short time after that, he rejected both his wife and Christianity, before dying suddenly in January or February 927.
It isn't recorded anywhere what Sigtryggr died of, and there's no record of his burial either. The ship burial in the book came out of my imagination. But who knows, maybe one day someone will unearth a ship burial near York, just like the one at Sutton Hoo in Suffolk. Then I will become famous for my prophecies!