Vampiric Gods in Frank Herbert's story

http://itunes.com/apps/Stanza. "Part 1": That first science-fiction
story, "Looking for Something," was published in the April 1952
issue of Startling Stories. It concerns the queer belief of a stage
hypnotist that the world he and everyone else sees is the illusion of
a master hypnotist. And of course it is. An ancient race of alien
beings hidden from sight by hypnotic command farms humans for a
glandular secretion that provides them with a kind of vampiric
immortality. Paul Marcus, the hypnotist, is discovered by the chief
indoctrinator of the aliens just as he is on the point of unearthing
the master hypnotic command buried in the mind of his pretty young
assistant. He is not killed; he is simply reprogrammed to another
fantasy life, as a street-car driver instead of a hypnotist. There he
will have no need for unsettling thought, the indoctrinator concludes
before going off to handle his next case.