Raj`a (الرجعة, romanized: āl rj'ah, lit. 'Return') (alternatively rjaa, raja'a, raj'at) in Islamic terminology, refers to the Second Coming, or the return to life of a given past historical figure after that person's physical death. While primarily used in Shia Muslim terminology, variations on the doctrine exist in Sunni Islam and the Bahá'í Faith, and raj`a is used to describe certain Christian doctrines in Arabic.
Raj’at or Raj’ah الرَجعَة means the return to this life after death. This return will be during the government of Imam Al-Mahdi (AS) over the whole world. Allah will bring back to this world the best believers to witness the victory of Ahlul Bayt (AS). Allah will also bring back to this world the worst enemies of Islam to see that and get some of the results of their evil deeds.
Raj’ah is mentioned in Quran (Sura Ghaafir, verse 11).
Yes, it is mentioned in many narrations that Imam al-Mahdi (AS) will martyred and Imam Husain (AS) will pray his Namaz e Janaza. (Biharul Anwar , volume 53, page 103 and Al-Kafi 8:250)
Some narrations about the raj'ah specifically mention the return of people from ancient times. Of course, not all narrations about the raj'ah are correct, and maybe those are not correct, but, at least, from what we have, it seems that the raj'ah will include people from all times and not only the time after certain Imams.
However, according to narrations, not all people return in the raj'ah. Only those who have the highest level of faith and highest level of depravity will be brought back to life.