Some mortal sorcerer calling himself the Necromancer. Gandalf was stunned when those words came out of Saruman's mouth. Had he been sampling the mushrooms he'd accused Radagast of favoring?
Why was the other Istar so convinced that the Necromancer could not be Sauron? Why was he so desperate to believe what he claimed was true that he would hold to that even in face of all the opposing evidence? Even Elrond had been willing to consider the possibility in face of the Morgul blade.
Gandalf snorted to himself. The Witch-king of Angmar would never serve a mortal. There was only one he served other than himself: the Maia who had approached him in a fair form, offering him a Ring of Power and thus enslaving him.
If the greatest of the Nazgûl had arisen again, why was it so far fetched his master had arisen again? The part of Gandalf who still remembered the days when he had been the Maia Olórin found it making complete sense. He knew his little brother's tenacity, (who better?) and knew nothing short of complete annihilation would stop him from pursuing the path he had chosen. For all that Saruman continued to claim the One Ring had washed down the Anduin to the sea, it was known it still endured. And while it did, Sauron would continue to endure, would continue to pursue the objectives he'd held sense the beginning of the Second Age: domination of Middle-earth.
Why did Saruman not see that? He too, had once known Sauron, back when they had once served Aulë together. True, they had never been friends: Gandalf could remember his little brother complaining viciously about his fellow Maia, and the bitter rivalry that had existed between the two. Gandalf secretly suspected that feud had been the reason Curumo had been willing to volunteer for the assignment to come to Middle-earth. But at the moment, he could sympathize with the complaints Mairon had always had.
The theory Saruman had been sampling those mushrooms from the Greenwood was looking more and more likely!
As always with my stories, Gandalf and Sauron are brothers. Everything else is canon(a mix of movie and book) and the mushroom line is, of course, from the new movie. It was too good to pass up using!