In addition to the twenty-one Conan stories that Howard actually wrote, there has been a plethora of Conan pastiches, written by various authors, in an effort to fill gaps in the Cimmerian’s career.
While Howard did leave a few very brief fragments and outlines for other Conan stories — which have been completed through “posthumous collaboration” — later writers have”revised” non-fantasy adventures to turn them into Conan stories, and have further diluted Howard’s Conan through a vast body of frank pastiches.
These are not Conan stories — not Robert E. Howard’s Conan — and have no more validity in relation to the stories than any Conan stories you might yourself decide to write. {…} It is a matter of spirit.

Karl Edward Wagner, forward to “Hour of the Dragon,” Berkley paperback, 1977.
It’s never been said better.