I read the book trilogy in paperback and perhaps they only were available in “bulk” paperback. They were from the 60's or 70's. I remember one book cover - an older man dressed as a typical wizard - long brown robe with hood, long grey hair and beard wearing a sword and holding a long cane or stick is sitting at some fellows kitchen table.
The wizard is from a different world and comes to ours. He returns to his bringing along a young man and woman. It’s the young man who owns the small house and kitchen table the wizard is sitting at - big shock to the young man when he turns on the light and sees this wizard character sitting there. At his home world the people are under attack by something like giant bats that sort of fly and float about. If they catch you they take away your ability to think. Eventually you’ll walk off a cliff or drown or starve since you aren’t human any longer.
So the people are fleeing to these giant, “keeps” their ancestors made long ago when these creatures first attacked humanity. Oh yes, the creatures/bats live in massive numbers deep underground and of course the wizard and our young man get to climb down a bazillion stairs into their nest for some important reason.