Imam Mahdi is the last savior of man from the descendant of Imam Husayn (pbuh).
“prophet Muhammad (PBUH) said about Imam Mahdi: “If only one day of this world remained, God would lengthen that day, till he raised up in it a man who belongs to me or to my family whose name is the same as mine and fill the earth with equity and justice as it has been filled with oppression and tyranny“
References: Sunan – Tirmidhi – Vo. 4, p. 438; Sunan – Abu Dawud, Hadith 4282 (in brief) and 4283
May God hasten the reappearance of Imam Mahdi