Doctor Faustus conjures up the image of Alexander the Great in response to a request by the German Emperor Carolus. Like many leaders throughout the centuries, Carolus is absolutely fascinated by Alexander, seeing him as one of the great figures of history, someone who provides a shining example for any self-respecting emperor to follow.
Despite the skepticism of one of the knights present, Faustus is indeed able to conjure up the spirit of the great Macedonian warrior-king. For good measure, he throws in the spirit of Alexander's paramour. The Emperor closely inspects the spirits and is well pleased with what he sees, so much so that he promises to give Faustus bounteous rewards.
As for the skeptical knight, he's furious to discover that Faustus has given him a pair of horns—though not for long, as Faustus, at Carolus's request, soon removes them. But he's made his point; just as Alexander once conquered the known world, Faustus feels able to conquer the unknown.