Risala: Ibn Fadlan's Account of the Rus (thanks to thesilvereel for pointing out this website)
I had come across Ibn Fadhlan before so I was quite intrigued when I saw 13th Warrior as the beginning of it was obviously based on his account of a Viking funeral.
The latter part of the film, with the Wendels being weird and scary in caves, is pure fantasy. For a start, there was no universal neolithic worship of a great mother goddess figure (ooh, there, I've said it now!), and if there had been, it wouldn't have involved sacrifice like that. Weird sacrificial practices were more of an Iron Age thing.
Still, it's a clever reworking of the story of Beowulf, with Grendel being the Wendels and Grendel's mother being the scary cave idol.