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Michael Crichton's novel Eaters of the Dead is partly fact and partly fiction, with the first half being Ibn Fadlan's account of the Norsemen in Russia and the second half telling how he was taken to Denmark to fight the Wendol, who are strongly implied to be leftover Neanderthals.
So what if the events depicted in the novel were real? Basically, WI relict Neanderthal populations survived in northern Europe, were advanced enough to have both metallurgy and cavalry, and ultimately lingered on into the Viking age, with Neanderthal raiders inspiring the story of "Beowulf"?
(From the perspective of the book, this IS OTL, with "Beowulf" being all that ultimately emerged, but let's take the POD and try to reach our own conclusions.)