One of the signs of the appearance of Imam Mahdi 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. The appearance of the Dajjal is even mentioned in the Testaments.
It is understood from the root of the word Dajjal (that means deception and lying) on the one hand and from hadith resources on the other hand, Dajjal is not exclusive to one person, but rather it is a title for a group of deceivers and cunning people who use any means to attract people to themselves.
One of the reasons for this claim is what we read in a famous hadith that the Prophet says,
“There has been no prophet after Noah but forewarned his nation from the conspiracy of the Dajjal and I also forewarn you about it!”[1]
It is clear that the previous prophets shouldn’t have warned their people about the conspiracy of the Dajjal who will appear at the End of Days when it is thousands of years ago. Especially, when the final parts of the abovementioned narration says,
“… Then the Prophet explained its characteristics for us and said, “Maybe some of those who have seen me or have heard my expression are alive in the time of the Dajjal!”
It indicates that after the Prophet some people who had the traits of the Dajjal would appear. This hadith may point to people who tried to deviate others from the way of the truth.
Consequently, in every different period of time some people exist who are titled as the Dajjal. It also is mentioned in some Islamic narrations that the Dajjals will appear before the advent of Imam Mahdi!
Anyway, the fact is that in any society, there are some deceiving people who try to mislead people, but, before the advent of Imam Mahdi activities of these Dajjals, who come one after another, increase in order to delay the advancement and improvement of the divine religion. However, it is highly probable that there is one great Dajjal who is above them all and supervises them.
[1] Sahih al-Tirmidhi, Chapter of what have been quoted about the Dajjal, p. 42.
