Who is al massih Dajjal?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Antichrist (Aldajal) is a man from Adam's offspring who will come before the Day of Judgment. Allah will give him extra ordinary powers, and He will make him a trial and affliction for people. He will claim to be God and will ask people to worship him instead of Allah. He will be a great trial for human beings. All the living people will follow him save those whom Allah has protected. The first people to follow him will be the Jews. He will stay on earth for 40 days. One of these days will be as long as a year; another day will be like a month and a third day will be like a week and all the remaining days will be like our days. In his last days, Jesus ( PBUH) son of Mary will come down from heaven and kill him. There are many sound Hadeeth describing him and describing his actions and deeds. So the Ahlu-Sunnah believe in his emergence.
Muslim scholars that have written about the signs of the DAY OF JUDGEMENT have made long accounts about Antichrist deducing their informations from the many Hadith about him.  All Muslims are asked to read about the signs of the Day of Judgment and about the trials and temptations of the last days, above all the Antichrist, as it is the greatest trial of them all. The prophet said: "There is nothing between the creation of Adam and the Day of Judgment greater than the Antichrist" In another narration it is said: "There is no trial between the creation of Adam and the HOUR greater than the Antichrist". This is why all messengers warned their people of the Antichrist, as said by the Prophet: "Each and every messenger sent by Allah warned his people against the liar and one-eyed Antichrist. He is undoubtedly one-eyed and your Lord is not one-eyed. It is written between his eyes: 'Kafeer' (disbeliever) that could be read by every believer".
So every Muslim should seek refuge with Allah asking Allah to protect him from all trials and temptations.
And Allah knows Best.