The Dajjaal will emerge at the end of time, so why did the Prophets warn their people about him? And when and where will he emerge?

Firstly: 

The greatest fitnah (trial, tribulation) on the face of the earth since the creation of Adam until the onset of the Hour will be the fitnah of the Dajjaal, as the Prophet (peace and blessings of Allaah be upon him) said. Hence there is no Prophet, from Nooh to Muhammad (may the blessings and peace of Allaah be upon them all), who did not warn his people about him, as an indication of have serious his fitnah will be. Although Allaah knows that he will not emerge until the end of time, nevertheless He commanded the Messengers to warn their people about him so that they would understand how serious the matter is. It was narrated that the Prophet (peace and blessings of Allaah be upon him) said: “If he emerges whilst I am still among you, then I will suffice you against him, otherwise each man should take care of himself, d Allah will take care of every Muslim on my behalf (and safeguard him against his evil).” Muslim, 5228. 

The Dajjaal is a serious matter, indeed he is the greatest fitnah, as it says in the hadeeth, since the creation of Adam and until the Hour begins. No wonder that he is the only one of the trials of life from which refuge is sought in prayer: “A’oodhu Billaah min ‘adhaab Jahannam wa min ‘adhaab il qabri wa min fitnat il-mahyaa wa’l-mamaat wa min fitnat il-maseeh il-dajjaal (I seek refuge with Allaah from the torment of Hell, the torment of the grave, the trials of life and death and the trial of the Dajjaal).” 

The word Dajjaal comes from the word dajl, which means misrepresentation or falsification, because he is a falsifier, indeed the greatest falsifier and the worst liar of all people. 

Secondly: 

The appearance of the Dajjaal is one of the signs of the Hour, but the time of his emergence is not clearly defined, because no one knows when the Hour will be except Allaah. Similarly with regard to the portents of the Hour, all we know is what has already happened (but with regard to those which have not yet happened, we do not know when they will happen).  The time of his appearance is not known, but we do know that it is one of the portents of the Hour. 

Thirdly: 

With regard to the direction from which he will emerge, he will emerge from the east, from the direction of tribulation and evil, as the Prophet (peace and blessings of Allaah be upon him) said: “Fitnah is here,” and he pointed towards the east. (al-Bukhaari, 3279; Muslim, 5167). So the east is the source of evil and tribulation, and he will emerge from the east, from Khurasaan, passing through Isfahan and entering the Arabian Peninsula from a point between Syria and Iraq, with no goal but Madeenah, because there lies the Bringer of glad tidings and warnings (peace and blessings of Allaah be upon him). He will want to destroy the people of Madeenah, but it will be forbidden to him, as was narrated from the Prophet (peace and blessings of Allaah be upon him): “At every gate thereof will be angels guarding it.” Narrated by al-Bukhaari, 1880; Muslim, 2449. This man will emerge between Syria and Iraq, and seventy thousand of the Jews of Isfahan will follow him, because they are his troops. (Narrated by Muslim, 5237). The Jews are among the most evil of the slaves of Allaah, and he is the most misguided of the slaves of Allaah. So they will follow him and support him, and they will be his troops, they and others who follow him. The Prophet (peace and blessings of Allaah be upon him) said: “O slaves of Allaah, stand firm; O slaves of Allaah, stand firm.” Narrated by Muslim, 5228. The Prophet (peace and blessings of Allaah be upon him) urged us to stand firm, because the matter is serious. And he (peace and blessings of Allaah be upon him) said: “Whoever hears of the Dajjaal, let him flee from him, for by Allaah, a man will come to him thinking that he is a believer, then he will follow him because of the specious arguments he puts forward.” Narrated by Abu Dawood, 3762; classed as saheeh by al-Albaani in Saheeh Abi Dawood.

 A man will come to him and say, “He will never lead me astray and I will never be affected by him,” but he will keep producing specious arguments until he follows him – we seek refuge with Allaah from that. We ask Allaah to protect us with His protection. Praise be to Allaah the Lord of the Worlds.