Superman gets a magical enemy with a lot of consonants in his name in issue 235 (March 1971) thanks to O’Neil, Swan and Anderson.
Ferlin Nyxly is a long time struggling musician, whose career suddenly takes off after he acquires a magical harp, which also begins to take over his personality. Despite the oddness of his name, and the use of magic in the story, there is no connection at all to Mr. Mxyzptlk.
Morgan Edge decides to launch a tv smear campaign against Superman, forcing Clark Kent to editorialize against himself. This is part of Edge’s larger plan, working with Darkseid.
So Superman has that to deal with, and the mysterious Sand Superman. When Nyxly dresses up as Pan and begins a crime spree, it’s almost too much for the hero. His loses his powers of flight and invulnerability, but is not sure whether Nyxly or the Sand Superman is the root of the problem.
Superman is almost killed by the magic from the harp, but the Sand Superman swoops on to save him, destroying the harp, which appears to kill Nyxly. Superman attempts to talk with the creature, but he simply heads away, mute.
Nyxly is not actually dead, and returns the following year, minus the harp.