Voldemort is the archenemy of Harry Potter, who according to a prophecy has "the power to vanquish the Dark Lord". Nearly every witch or wizard dares not utter his unmentionable name, and refers to him instead with such expressions as "You-Know-Who", "He-Who-Must-Not-Be-Named" or "the Dark Lord". Voldemort's obsession with blood purity signifies his aim to rid the wizarding world of Muggle (non-magical) heritage and to conquer both worlds, Muggle and wizarding, to achieve pure-blood dominance. Through his mother's family, he is the last descendant of wizard Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry. He is the leader of the Death Eaters, a group of evil wizards and witches dedicated to ridding the Wizarding World of Muggles and establishing Voldemort as its supreme ruler.

Harry potter the name that cannot be said? Okie... think carefully first, then make a decision:
The relevant sentence in the passage is: Nearly every witch or wizard dares not utter his unmentionable name, and refers to him instead with such expressions as "You-Know-Who", "He-Who-Must-Not-Be-Named" or "the Dark Lord". So, the answer is Voldemort.