Dajjal is the last leader of falsehood, will come before the reappearance of Imam Al-Mahdi (AS). He will be killed by the last leader of righteous, Imam Al-Mahdi (A.S.). Dajjal is one of the definite signs of the reappearance of Imam Al Mahdi (A.S.). Dajjal will lead an army of the enemies of Ahlulbayt and fight against Imam Al-Mahdi (A.S.) and he will be defeated. (Al Amali by Al Toosi, Page 459). It is mentioned in Kamaal Ad-Din Page 336 from Imam Jafar As-Sadiq (A.S.) that Imam Al-Mahdi (A.S.) will kill Dajjal. Unfortunately, we read narrations in some Sunni books, which are taken from Non-Muslims and it go against the Truth. For example, the narration in Sahih Muslim which is called Hadeeth Al-Jassasa Volume 8 Page 204 & Hadeeth in Sahih Al Bukhari about Ibn Sayyad who is according to the narration of Bukhari the Dajjal. Ibn Hajar Al-Asqalani in Fath Al-Bari Volume 13 Page 276 confirmed his opinion that Dajjal is still alive. We don’t have in our authentic narrations the exact date when Dajjal was born and in fact we don't need it, but we only need to know that Dajjal is a devil satanic leader and he will lead his followers to the hellfire, and he will fight against Imam Al-Mahdi (A.S.) and he will be killed by Him. (Al Amali by Al-Toosi Page 527, Al- Bihar Volume 76 Page 74, Al-Waafi Volume 26 Page 186).