Asalaamu Alaikum Wr Wb, here is some important information on Ya'juj Ma'juj:
Ya'juj Ma'juj (Gog and Magog)
The emergence of the mighty tribes of Ya'juj and Ma'juj is also a major sign of Qiyamah.
Allah says in the Qur'an, "When Ya'juj and Ma'juj are let loose (from their Barrier) and they swiftly swarm from every mound" (Surah Al-Anbiya)
Allah says in the Qur'an,
" (Zulqarnain) said, 'This is a mercy from my Lord, but when the promise of my Lord comes, He shall level it down (the barrier) to the ground. And the promise of my Lord is ever true." (Surah Al-Kahf)
"Once the Prophet awoke from such a sleep that his face was red and these words were on his tongue, "There is none worthy of worship but Allah. Destruction is upon the Arabs on account of that evil which has come close to them. Today a hole as big as this has opened in the wall of Yajuj and Ma'juj. (The Prophet indicated the size of the hole with his finger and thumb)" (Bukhari/Muslim)
In the lengthy Hadith of Nawwas-b-Saman it has been mentioned, "And Allah will send Yajuj and Ma'juj and they will come from every lofty place." (Muslim)
Ya'juj Ma'juj and Zulqarnain
Many thousands of years ago the barbarous tribes of Yajuj and Ma'juj were imprisoned behind an iron wall built by Zulqarnayn. Referred to in the Qur'an in Surah Al-Kahf. Zulqarnayn was a Muslim Arab (fathul bari) who lived at the time of the Prophet Ibraheem and not Alexander the Great as it is commonly known. From amongst one of the four that ruled the entire world (the other three being Prophet Sulayman, Nimrod & Buktnasr) he was a pious and just king, provide with all forms of strength through which he was able to carry his conquests and missions.
Once he carried a mission in three directions, the far west, far east, and then in a northerly direction. Travelling first in the westerly direction, he conquered the lands he passed through establishing the laws of Allah therein until he reached the setting of the sun. There he met people that didn't believe in Allah. Given a choice of punishing them for their kufr or being lenient by inviting them first to Islam; he chose the latter and addressed them, ''Those evil-doers who do injustice to themselves by rejecting Allah will be punished by death in this world and the hereafter, the fire of hell is their abode. As for those who accept the invitation and believe in Allah they will be treated leniently and in the hereafter Jannah is their place of rest.''
After the journey towards the west he made preparation for the journey towards the east. Conquering the lands he passed through, establishing he laws of Allah therein. He continued travelling in the easterly direction until he reached the rising of the sun. In this area of the east he saw a nation receiving the sunshine without any obstruction and they were dealt with like the previous people in the west.
After the journey to the east he started his northern Journey, he kept on travelling until he reached the midst of two mountains, it was here he came across a tribe who complained to him about the tribes of Ya'juj and Ma'juj.
Ya'juj and Ma'juj inhabited the land behind the mountains, plundered them, committed bloodshed, and then ran away. Observing Zulqarnain's power they asked to set a barrier in return for a wage for their protection from the disaster and bloodshed, which they often bore at the hands of Ya'juj Ma'juj. Refusing to accept any wealth he built an iron wall with their help which Ya'juj Ma'juj could not cross or pierce. (Qurtubi/ Ibne-Kathir)
The wall of Zulqarnain
Many different opinions have been expressed as to the location of the iron wall of Zulqarnain. Having read many books, the best on the subject I came across was Sheikh Hifzur-Rahman's "Stories of the Qur'an" and thus have chosen to share a brief summary of what he has written.
"The Yajuj Ma'juj caused destruction and blood- shed in a vast area, and because of their oppression many barriers and walls were erected in different times and places by different kings. Four being the most famous:
1) The Great Wall of China which was built by the Chinese King Fagfor 3460 years after Prophet Adam was put on the Earth.
2) The wall in central Asia near Bukhara and Tirmidh in a place called Derbent.
3) The wall in Dagistan Russia also known as Derbent near the Caspian sea.
4) The wall which is in the westerly direction to the third in the region of the Caucasus.
Because these walls were built for one purpose and are all situated in the North, it has always been very difficult to determine exactly the wall built by Zulqarnain."
He further writes,
"The biggest out of the four is the great wall of China and nobody is of the opinion that this is the wall built by Zulqarnain as it is in the easterly direction while the Qur'an indicates the wall of Zulqarnain is in the Northerly direction."
Thus leaving walls 2, 3, and 4.
"Historians like Masoodi, Istakhari and Hamawi are of the opinion that the wall of Zulqamain is wall number 3 or 4. Those that have said it is wall number two have confused the issue due to the location of Derbent which is near Bukhara and also in Degistan.''
He finally writes,
''Out of two, the historians are of the opinion that it is wall 3 or 4, the master of hadith Allama Anwar Shah Kashmiri in his book Aqeeda-tul-Islam holds the opinion that the wall of Zulqarnain is wall number 4, the one in the region of Caucasus.' (Stories of the Qu'ran)
After this short summary I find myself inclined to Allama Aloosi's opinion I conclude with his words, ''We do not know the location of this wall and it is very probable that great seas and mountains stand between us and the wall, and between Ya'juj Ma'juj and the rest of the world.''
Trying to break free
Abu Huraira (R.A.) narrated that every day Ya'juj Ma'juj try to break free through the wall of Zulqarnain until they reach the end of it to the extent that they could actually see the light on the other side. They then return home saying, ''We will break through tomorrow.'' However, Allah causes the wall to revert to its original thickness and the next day they start all over again. This process continues each day as long as Allah wills for them to remain imprisoned. When Allah wishes for them to be released, then at the end of that day they will say, ''If Allah wills, we will break through tomorrow.'' The next day they will find the wall as they left it the previous day and after breaking through the rest they will escape. (Ahmed, Tirmidhi, Ibne Majah)
Some ulama have written that this hadith is weak and has been taken from the Jews as it contradicts with the verse from the Qur'an, ''They are not able to cross it or pierce it.'' (Surah Al-Kahf)
However, if found that it is a Marfoo hadith, it still does not contradict the Qur'an as the Qur'an is referring to the time Zulqarnain built the wall and secondly the word ''naqb''
means a complete hole through which they are able to break free.
The great master of Hadith Ibne Hajr al-Askalani giving reference of Ibne Hibban and Abd- b-Humaid has mentioned this Hadith and not raised any doubt or indicated that this hadith is not Marfoo but positively mentioned that Ibne Arabi has said three miracles are evident from this Hadith and then mentioned the three miracles:
1) It never occurs to these tribes that they must continue work throughout the night. After all, they are in such large numbers that they can easily delegate the work amongst themselves and work in shifts. However, Allah does not allow this thought to occur to them.
2) It does not occur to them that they can merely cross the mountain or scale the wall, which they can do through the aid of equipment they possess in large numbers. According to a narration by Wahb-b-Munabbah it is known that these tribes are agriculturists and artisans possessing various types of equipment.
3) The thought of saying, "Insha-allah (if Allah wills)" never enters their minds and it will only occur to them when Allah wills that they be released. (Fathul-Bari)
Many myths surround Ya'juj and Ma'juj. One being that the common people believe that Ya'juj and Ma'juj are not human beings but some kind of third being. How interesting it may be to believe that they are a third being, the reality is that they are human beings just like yourself and I, constitute nine-tenths of mankind and are from the progeny of Prophet Nuh.
The Holy Prophet said,
"Ya'juj and Ma'juj are the children of Prophet Adam and not one will die until thousand are born to him.'' (Abdullah-b-Amr/Fathul-Bari).
The Messenger of Allah said,
"Allah divided mankind into ten parts. Nine tenths constitute Ya'juj and Ma'juj while the remaining one tenth constitutes the rest of mankind." (Abdullah-b-Amr/Fathul-Bari).
Hadrat Abu Saeed Khudri narrates that the Messenger of Allah said,
"On the Day of Judgement, Allah will tell Adamto pick out the Jahanamis from his progeny. Adam will ask, "O Allah, who are they?" Allah will say,"999 out of 1000 are Jahanamis while the one is a Jannati." On hearing this the Sahaba over taken by fear asked, "Who will the ONE Jannati be?" The Prophet replied, "Do not grieve the 999 will be Yajuj Ma'juj while you will be the one Jannati." (Bukhari/Muslim)
A second myth is that people believe that Ya'juj and Ma'juj were created from Adam without Eve (Hawa) on the basis of Ka'ab's narration, "They are from the children of Adam. That is Adam had a wet dream, the semen intermingled with the earth and they were created from that Earth."(Fathul-Bari)
It should be noted that this narration is very weak and objectionable as it is proven from many Ahadith that the Prophets of Allah do not have wet dreams because the wet dream is from the devil and the Prophets of Allah are protected from the devil.
Secondly there is a hadith which can be found in Fathul-Bari that clearly states that they are from the progeny of Prophet Nuh. The Prophet Nuh was definitely from the children of Adam and Eve.
Physical Appearance of Ya'juj and Ma'juj
There are many different narration's regarding their appearance. These different narration's suggest that Yajuj and Ma'juj are of three types.
1) Some are as tall as the tree Arz - that is 120 ft.
2) Some are four arm lengths tall and four arm lengths wide.
3) Some spread one ear to sleep on and cover themselves with the other. Some also state that they are two spans tall and the tallest amongst them are three spans. (Fathul-Bari)
It should be noted that all the narration's above are weak and should not be taken. The authentic Ahadith like the Hadith of Nawwas-b- Saman in Muslim suggests that they are very strong and powerful people whom nobody has the power to fight. It has also been narrated that they have wide faces, small eyes, grey hair and their faces are like shields covered with skin. (Musnad Ahmad)
Emergence of Ya'juj Ma'juj before Qiyamah
A short period after killing the Dajjal, Prophet Jesus will be informed of the release of Ya'juj and Ma'juj. Obeying Allah's command He will take the Muslims to Mount Tur for their protection as nobody will be able to kill the Ya'juj Ma'juj. Ya'juj and Ma'juj will come forth from every lofty place causing destruction and bloodshed wherever they go. Their first batch will come to lake Tiberias and drink all of its water. Their last batch will come to it and say, "There was once water in this lake."
Marching on, carrying out pillage and murder to their left and right they will come to mount of Khamr (a mountain in Jerusalem)
"We have killed the inhbitant of the Earth, let us now kill the inhabitants of heaven." Thus they will shoot their arrows towards the sky. Allah will return the arrows covered with blood and these fools will think they have killed those in the heaven. Mean while Prophet Jesus and his companions will remain confined until the head of an ox becomes more valuable than 100 gold coins. Prophet Jesus and his companions will pray for their destruction. Allah, answering their prayer will send insects upon the necks of Ya'juj Ma'juj and in the morning they will be found dead like one dead man. In the narration of Abu Saeed al- Khudri which can be found in Tadkhira of Imam Qurtubi, it has been mentioned; the believers not being able to hear their noise that morning will say "Will anybody sacrifice his life and see what the situation is." Volunteering, a believer will come down the mountain thinking that he will never return . However to his suprise he will find that they are all dead and he will shout, "Good news! Your enemy has died,"
Prophet Jesus and his companions will come down but will find that the earth is full with their stinking corpses. Yet again Prophet Jesus and his companions will turn to Allah and pray. Allah will send birds with necks like the necks of Bactarian camels which will carry the corpses and throw them where Allah wishes. Thereafter, Allah will send rain and the earth will be cleaned. (Nawwas-b-Samaan/Muslim)
Read more on the Major Signs before the Day of Judgement here:http://www.inter-islam.org/faith/Majorsigns.html#ma'juj