"Under the Keeper of the Key" is a short Cthulhu Mythos story by Dutch author Jaap Boekestein. It was published in the James Ward Kirk Publishing anthology Lovecraft after Dark in November 2015.
Synopsis[]
After many years of corresponding with the mysterious Machteld, Gerald Voss is finally invited to a meeting where he will learn the secrets of Great Cthulhu and the Old Ones. Participating in a ritual designed to heighten his consciousness through pain and pleasure, he is ultimately transformed by Machteld into an amphibious creature like herself, and ventures forth to claim the knowledge which he has earned.