The Wizard Merlin plays a minor, but not secondary, role in Ludovico Ariosto's Orlando Furioso insofar as he, through his fairy assistant Melissa, foretells and prepares the glory of the Este family, the sponsor of the poem. Torquato Tasso, on the contrary, didn't like at all that such a heathen magician should be exalted as a prophet in Christian literature.
But things are more complex than this. A traditional Christian writer, CS Lewis, in his 1943 novel That Hideous Strength describes Merlin as a true believer and a sage of the late Latin Era, just needing to be 'handled with care' since he is the son of a civilization in which the boundaries between good and evil were more flexible than nowadays. In fact, the villains in the novel, i.e. those who are trying to use "that hideous strength," look for the 'risen' Merlin in order to have him on their side, but he -- on his head -- will immediately side with the heroes, and will prove of great help for their victory.
In the early 1940s HP Lovecraft's story The Case of Charles Dexter Ward was first published too, in which Merlin can be identified with the mysterious "118," the code name of an old and ancient personage who is conjured -- very fittingly, if by mistake! -- by Doctor Willett the vampire hunter. And precisely "118" will turn out to be the main cause of the defeat of the necromancer Joseph Curwen and his dangerous friends. In his recent graphic novel based on Lovecraft's story, INJ Culbard adds a witty key: the presumed Merlin (picture above) has a face that mirrors Willett's.
Interestingly enough, in JRR Tolkien's unfinished poem The Fall of Arthur, Merlin is not even mentioned. [The whole poem is being translated into Italian on this blog; click here below on the tag "The Fall of Arthur" to see the episodes that have already been posted.]