National Geographic is running a piece on Peter Dendle's Demon Possession in Anglo-Saxon England (which I've not yet read). I'm very interested in reading this book because it strikes me as totally wrong, so I'm wondering where Dendle's coming from.
First off, if their leechbooks are any indicator (and I'd say they are), the Anglo-Saxons certainly thought that evil or mischievous spirits caused sickness and madness. In addition to afflictions from being elfshot, people could suffer from "fiendsickness" or "devilsickness." Hagiography had examples of people being tormented by demons or devils, and of course the idea of demonic forces working in our world likely first came to England long before even the Anglo-Saxons did, with the conversion of the Britons. The only way I can see this working out is to have an extremely narrow definition of "possession."
Even then, I'm skeptical of this claim. Let's say that we define "demon" and "possession" very narrowly -- i.e. demon means "evil spirit in league with the Christian Satan" and possession means "absolute control over every element of a person's body." Even then, this is a startling claim to make from negative evidence. Let's say we have no extant examples of such demonic possession from Anglo-Saxon England (we may not under such a narrow definition) -- does this then mean that they did not have demonic possession? That's a pretty big claim to make for a period with limited manuscript evidence.
Furthermore, the article seems to indicate that we do have manuscript evidence of demonic possession, in this 50-year outbreak in Northumbria. It strikes me that this is less likely an "outbreak," and more likely simply one of the few records that survived the dissolution of the monasteries and the fire at the Cotton Library.
Maybe Dendle has some stronger evidence than was introduced in the article, such as Anglo-Saxons seeming to consider demon possession foreign or explicitly referring to it as "Irish" or "Merovingian." Dendle has done other work on demons and devils, so I'm betting he's got more powerful ammunition than this popular article shows us.
h/t Scribal Terror