Unlocked Wordhoard brings up some very convincing objections to Peter Dendle's apparent contention that demonic possession was not a part of Angl0-Saxon belief systems and didn't become "popular" in England until the Normans imported it in 1066:
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.
I would just add, Dendle says, "There is no reference to a contemporary Anglo-Saxon case of possession for 300 years." If this means he's limiting allowable evidence to court cases and other legal documents like deeds and wills while excluding things like charms and leechbooks (medical books), I think he may be fudging the evidence -- perhaps because he wants to make a "statement." The National Geographic article says that Dendle "draws parallels between the phenomenon in medieval England and its resurgence in the West in recent decades," which I didn't mention in my original post because I thought it was probably hogwash and I'm deeply suspicious of scholars with extracurricular agendas. Now it looks like somebody's going to have to read the book.