For example:
. Hazrath SubanRZ narrates that Hazrat Rasool AllahSLM said "For your treasures (Caliphate) three persons will fight but none will overcome it, then black flags will rise from the east, they will massacre you {Halaku Khan's Army killed 1.6 million Muslims in Baghdad alone} in such a way that no group has done it ever before. After this Khalifatullah Mehdi will arrive, when you hear about Mehdi reach him and pledge allegiance to him even if you have to crawl over snow because he is Caliph of Allah the Mehdi" (Ibne Maja, Ahmed bin Hanbal, Haakim, Delami, Ruyani, and Nayeem bin Hammad)