One of the signs of the reappearance of Imam Mahdi (a) is the coming of the Dajjal. In Sunni sources the advent of the Dajjal is considered to be a sign of the Day of Judgment. (Sunan Tirmidi, vol.4, pp.507-519; Sunan Abi-Dawud, v.4, p.115; Sahih Muslim, v.18, pp.46 and 81). Shia sources state that he is a sign of both the Day of Judgment and the reappearance of the Living Imam (a). (Bihar al-Anwar, v.6, p.296)
Unusual traits have been attributed to him in some traditions, such as: he falsely claims his lordship (Sunan Ibn Majah, v.2, p. 1360), is old (Sahih Muslim, v.8, p. 205), carries fire and water (Sahih Bukhari, v.8, p. 103), and cures the blind and those suffering from leprosy (Musnad Ahmad, v.5, p. 13).
The commander of the faithful, Imam Ali (as) was busy delivering a sermon when he repeated three times: “Ask me (all that you need) before you lose me.” A righteous individual by the name of Sa’sa’ah ibn Sawhan stood up and asked: “Oh commander of the faithful! Who is the Dajjal?” In response to this question, Imam Ali (as) replied: “Be aware that the (name) of the dajjal is Sa’ed ibnul-Sa’ed. The unfortunate one is he who acknowledges and accepts him, and the fortunate being the one who rejects him. He will emerge from a place called Isbahan, from an area which is known to be the area of the Jews. His right eye is shut, bearing no eyeball. His other eye is in his forehead, shining so brightly that it resembles the morning star in brightness. In his eye there is a piece of meat that seems to have mixed with some blood and on his forehead, in between the two eyes, it has been written ‘Kafir’ in a way that every literate and illiterate person can read. He travels over the oceans and the sun accompanies him. In front of him, there is a mountain of smoke and behind him there is a white one that people perceive to be food. He will appear during a very hard drought. He will be riding a dark or green donkey. Every step this donkey takes is equal to one mile. He will travel and go here and there on earth. Every water source or spring that he crosses will turn and remain dry till the Day of Judgment. He will shout with a voice heard by all in the East and the West saying: “Come to me, Oh friends and followers of mine!” I (am a God) who has created, who has created the faces, has measured every persons portion (of sustenance), and is your guide. I am the exalted Lord!” He is a liar! He is Allah’s (swt) enemy…and in the end he will eventually be killed by Jesus (a), who prays behind Imam Mahdi (a).” (Lotfollah Safi, Muntakhabul-Athar, p. 532)
Regarding the reality of the Dajjal nothing concrete can be said. But, there are a few possibilities derived from religious sources, such as:
First, the dajjal is a person uses witchcraft and will appear at the end of time. He will be the head of a huge deviant army. Some traditions name him “the messiah of misguidance” which is the opposite of “the messiah of guidance”, meaning Prophet Jesus (a). During the reappearance of Imam Mahdi (as), he will begin to work against him. He is of those who have long lives and is and will remain alive until the day he appears. He will take over the whole world expect for Makkah and Madinah. (Tahdhib, v.6, p. 12)
Second, Iblis or Satan is the dajjal.
Third, The Sufyani is the dajjal.
Fourth, The dajjal “symbolizes the highest levels of civilisation, culture and Western technology that are in conflict with Islam and its principles; a culture that tries to conquer and enslave all. We can clearly see today how the West’s materialist culture and technology has taken over all societies today, even Muslim ones…” (Seyyid Muhammad Sadiq, Tarikh ma badal-Dhuhur, pp. 141 and 142. Quoted and obtained from: Ali Asghar Rizvani, Maw’ud Shenasi, p. 533)
Derived from an article on Islam Quest