Please explain the status of MAHDI in Islam in the context of Qur'an and Sunnah?

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.

It is stated in some sound Hadith that Allah will send a Caliph in the final days. He will be a just ruler. He will belong to the Prophet's family. His name will be like the name of the Prophet Muhammad (Blessings and peace of Allah be upon him) and his father's name will be the same as the name of Prophet's father. There are many Hadith describing his natural features. He will fill the earth with justice after it has been filled with injustice and aggression. There are many sound Hadith stating that al-Mahdi will appear, narrated by more than sixteen companions and complied by many scholars of Hadith. Some of these Hadith are as follows: Ali () narrated that the Prophet (Blessings and peace of Allah be upon him) said: "If it does not remain for this world but one day, surely Allah will send a man from my family who will fill the earth with justice after it has been filled with injustice and aggression". Related by Ahmad & Abu Dawood. Abu Said al-Khudri reported that Prophet (Blessings and peace of Allah be upon him) said: " al-Mahdi is from me. He has a large front and a hooked nose. He will fill the earth with justice after it has been filled with unfairness and injustice. He will rule for seven years". Reported by Abu Dawood. So far, he has not appeared as Imam Ibn Katheer mentioned in his book (al-Fitan wa al-Malahim). He will appear in the same period as Isa (Christ) will descend as mentioned in the Hadith of Jabir who said: "Isa Ibn Maryam (Christ) will descend and their Emir (Leader) says: Come and lead us in our prayer. He will reply: "No, Allah has honored this Ummah by putting some of you Emir over the others." Imam Ibn al-Qayyim said 'This Hadith chain is good.' The Islamic sects differ in their beliefs about al-Mahdi : 1 -The faith of Ahlus-Sunnah wa al-Jama'a is the same as mentioned in Hadith. They believe that he will be a good and just ruler and an originator in this religion. 2 -The Shi'a al-Imamiyah believe that their last Imam, the twelfth is al-Mahdi. His name is Mohammad son of al-Hassan al-Askari. They believe that he is from the progeny of Hussain not from the progeny of Hassan () as the Ahlus-Sunnah wa al-Jama'a believe. They believe also that he is still alive and exists. He entered in the cave of Samurra before more than one thousand and one hundred years ago at the age of five years. They believe that he exists everywhere but no one can see him. There is no evidence of Qur'an or Hadith or from logic supports these claims. 3 -Some Muslims deny the appearance of al-Mahdi. Some Muslims who have a poor knowledge of Hadith and who do not know how to distinguish between the correct and incorrect Hadith, deny the coming of al-Mahdi. But the sound Hadith refute them. Allah knows best.