You arrive at Mr. X's tower, fighting through waves of Syndicate henchmen to reach the mastermind of it all himself, Mr. X. "Mr. X" gestured for his minions to wear you down, and doesn't seem to expect them to actually defeat you. After you prevail, "Mr. X" maliciously laughs as he stands up from his throne and combusts, revealing his true form as a robot clone. He's not Mr. X, he's a robot clone of the real deal and now you'll have to fight him in order to proceed further.