Praise be to Allah.
The appearance of the Mahdi at the end of time, when he will fill the earth with justice and fairness after it had been filled with injustice and wrongdoing, is something that is mentioned in the saheeh Sunnah, and some of the scholars of hadeeth said: The ahaadeeth about the Mahdi are mutawaatir because of their large number, and the descent of ‘Eesa ibn Maryam (peace and blessings be upon him and our Prophet) and his praying behind him is something that is well known. The name of the Mahdi and the name of his father will be like the name of the Prophet (peace and blessings of Allaah be upon him) and the name of his father; he will be one of the descendents of Faatimah (may Allaah be pleased with her).
It should be noted that this Mahdi who is mentioned in the ahaadeeth is not the so-called awaited Mahdi mentioned by that deviant group, the Ithna’ ‘Ashari (Shi’ah) who claim that he entered the tunnel and is awaiting the order to emerge, and whose name they say is Muhammad ibn al-Hasan al-‘Askari. This is an illusion and a myth, and the refutation of that is discussed in books, and there is no room to go into it here.
But we advise you to read Minhaaj al-Sunnah by Shaykh al-Islam Ibn Taymiyah and summaries of it such as Manhaj al-I’tidaal by al-Dhahabi, and Tuhfat al-Ithna ‘Ashariyyah by Muhibb al-Deen al-Khateeb. It is essential to note that every group is awaiting a Mahdi such as the Jews who are awaiting the Mahdi, but for them he will be the Dajjaal who will be followed by the Jews of Isfahaan. The Christians are awaiting a Mahdi which is the descent of the Messiah (Second Coming of Christ). We Muslims have no doubt about the descent of ‘Eesa ibn Maryam (peace be upon him), when he will break the cross and kill the pigs, and will abolish jizyah and rule according to the sharee’ah of Islam, and at that time there will be no war.
Throughout history there have appeared liars who claimed to be the Mahdi. Some historians have counted them and said that there were more than thirty of them, including Ibn Tumart in Morocco and the Mahdi of the Sudan, etc.
As for the view that he exists at present and that he has been born, this is merely a claim that is made by some people, but there is no evidence for it. It is most likely that it is a myth. The ahaadeeth that have been narrated about the Mahdi do not mention where he will be born or when, and they do not tell us to look for him or to bring him up, rather they say that Allaah will prepare him in a single night and that the Muslims will swear allegiance to him as their caliph and will be united behind him.
When will that happen?
Only Allaah knows.
And Allaah knows best.