Simon Morgenstern is both a pseudonym and a narrative device invented by William Goldman to add another layer to his novel The Princess Bride. He presents his novel as being an abridged version of a work by the fictional Morgenstern, an author from the equally fictional country of Florin.
The details of Goldman's life given in the introduction and commentary for The Princess Bride are also largely fictional. For instance, he claims that he was inspired to abridge Morgenstern's The Princess Bride for his only child, a son, when the book actually originated as a bedtime story Goldman wrote for his two daughters. He not only treats Morgenstern and the countries of Florin and Guilder as real, but even claims that his own father was Florinese and had emigrated to America.