After Such Knowledge Trilogy, for pleasure, I have found a parallel not to Poul and Karen Anderson's The King Of Ys Tetralogy but to Poul Anderson's Is There Life On Other Worlds? Blish's character, Hess, says:
"'...I was interested in the history of science. That involves trying to understand why there wasn't any science for so long, and why it went into eclipse almost every time it was rediscovered.'"
-After Such Knowledge (London, 1991), p. 418.
Hess theorizes that:
the mind fears accumulated knowledge;
when it has accumulated too much knowledge, it panics;
then it invents mystical reasons to return to the Dark Ages;
having engineered World War III, his employer, Baines, and he have conjured a hallucination of demons to remove their own guilt;
they are all insane.
Hess is swallowed by a demon.
Anderson analyzes the historical accidents that caused the long delayed birth of science. See here and here. And in the concluding section of The Shield Of Time, he dramatizes alterations to medieval history that would have prevented the emergence of science and of social and political freedoms. Anderson does not bring literal demons on-stage but his account is fearsome enough, with the Inquisition still active in twentieth century Europe.