You mean that phantasm that "Bible scholar" Joel Rosenberg write about in his fiction?
Phantasm? hardly, and Joel Rosenberg is hardly the first person to mention it.
“The Mahdi” is an integral part of Islam, and every so often throughout History, someone has stood up and claimed to be the Islamic Mahdi and touched off a fresh round of Jihad as a result.
While we were fighting our civil war here in America, “the Mahdi” arose in what was then still a part of the Ottoman Empire in the area known as Soudan, and his jihadists fought their way down into the heart of Aftica, slaughtering thousands of natives and others in the process.
Ayatollah Khomeni never claimed to be the Mahdi, but he didn’t stop others from calling him “Imam” either. In the end, just another fakir...
By the way, have you read Rosenberg’s latest?? I burned through it in about three days, and it is well sourced like his other writings are, and makes an excellent read while providing ample food for thought.