I'm sorry to interfere.
We in Slovene say "mora". I checked the SSKJ dictionary and there's a very interesting explanation of the word "mora" when meaning "a nightmare". It says (translated):
"after folk believing mora is a creature who chokes a human being and harms to animals."
This creature doesn't belong to the Christian belief, so it must be older, from the old Slavic belief. Can you trace it out also in Slovak and Czech?