I want know about the Yajuj and Majuj (Gog and Magog). I want to know that when will they come, what will they do, where are they now. Please answer my question with the details.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Yajuj and Majuj (Gog and Magog) are two populated nations. They are from the offspring of Adam as it is proved in Al-Bukhari and Muslim.

The Prophet said: “Allah Almighty will say on the Day of Resurrection, ‘O Adam!’ Adam will reply, ‘I am at Your service, and all good is in Your Hand.’ Allah will say, ‘Get up, and take out the people of the Hellfire from among your descendants.’ Adam will say, ‘O Allah, and who are the people of the Hellfire?’ Allah will say, ‘From every one thousand, take out nine-hundred-and-ninety-nine to the Fire and one to Paradise.’ Thereupon, children will become hoary-headed, every pregnant female will have a miscarriage, and one will see mankind as drunken, yet they will not be drunken but dreadful will be the Wrath of Allah.' This was hard on them (the Companions of the Prophet ), so they asked, 'O Messenger of Allah! Which one of us is that one (who will be taken to Paradise)?' He said, 'Rejoice, for it is one from you and one thousand from Gog and Magog.'

Allah has mentioned in the Quran that the barrier raised by Dhul-Qarnayn (a righteous and powerful man) prevents them from getting out. If they get out, they will destroy crops, cattle and progeny.

Allah Says (what means): {Until, when he reached [a pass] between two mountains, he found beside them a people who could hardly understand [his] speech. They said, “O Dhul-Qarnayn, indeed Gog and Magog are [great] corrupters in the land. So may we assign for you an expenditure that you might make between us and them a barrier?” He said, “That in which my Lord has established me is better [than what you offer], but assist me with strength; I will make between you and them a dam. Bring me sheets of iron” - until, when he had leveled [them] between the two mountain walls, he said, “Blow [with bellows],” until when he had made it [like] fire, he said, “Bring me, that I may pour over it molten copper.” So Gog and Magog were unable to pass over it, nor were they able [to effect] in it any penetration.} [Quran 18: 93-97]

Allah and His Messenger have stated that the appearance of Yajuj and Majuj is among the signs of the Day of Resurrection.

Allah Says (what means): {[Dhul-Qarnayn] said, “This is a mercy from my Lord; but when the promise of my Lord comes, He will make it level, and ever is the promise of my Lord true.”} [Quran 18: 98]

At that time, they will appear like the waves of ocean. This will take place just before the Day of Resurrection.

Allah describes their condition when they will get out from behind the barrier. Allah Says (what means): {Until when [the dam of] Gog and Magog has been opened and they, from every elevation, descend. And [when] the true promise has approached; then suddenly the eyes of those who disbelieved will be staring [in horror, while they say], “O woe to us; we had been unmindful of this; rather, we were wrongdoers.”} [Quran 21: 96-97]

Besides, Prophet Muhammad has informed that a hole through the barrier of Gog and Magog has been made.

Zaynab bint Jahsh narrated that one day the Prophet came out in a state of excitement with his face quite red, and he said: “There is no God but Allah. There is a destruction in store for Arabia because of the turmoil which is near at hand as the barrier of Gog and Magog has been opened like it; and he (in order to explain it) made a ring with the help of his thumb and forefinger. I said: “O Allah's Messenger! Would we be destroyed despite the fact that there would be pious people among us? He said: “Yes, when the evil would predominate.” [Al-Bukhaari and Muslim]

They (Gog and Magog) will appear after the descent of the Prophet ‘Eesa who will kill the Dajjal.

An-Nawaas Ibn Sam'aan narrated in a very long Hadeeth: “… Then a people whom Allah will have protected from the Dajjaal will come to Jesus, son of Mary, and he will wipe over their faces and inform them of their ranks in Paradise. It will be under such conditions that Allah will reveal to Jesus these words: I have brought forth from among My servants such people against whom none will be able to fight; you take these people (the believers) safely to Mount Toor; and then Allah will send Gog and Magog and they will swarm down from every slope. The first of them will pass the lake of Tiberias and drink out of it. And when the last of them passes, he will say: There was once water there… Jesus and his companions will then be besieged there so that the head of an ox will be dearer to them than one hundred Dinaars are to you, and Allah’s Messenger, Jesus, and his companions will supplicate Allah, Who will send insects to them (Gog and Magog), which attack their necks; and in the morning they will perish like one single person. Allah’s Messenger, Jesus, and his companions will then come down to the earth and they will not find on the earth as much space as a single span which is not filled with their putrefaction and stench. Jesus and his companions will then again beseech Allah, Who will send birds whose necks are like those of Bactrian camels, and they will carry them and cast them wherever Allah wills. Then Allah will send rain which no house of clay or tent of camel hair keeps out, and it will wash clean the earth until it looks like a mirror. Then the earth will be told to bring forth its fruit and restore its blessing.” [Muslim]

Finally, although no one knows where they are living now, but there is no doubt in their existence on this earth.

For more benefit, please refer to Fataawa 361294, 350389, 280567, 91399, 806, and 12031.

Allah knows best.