Mafateeh AlJinaan p.235 dua on the third day of Shaban speaking about Hussain a.s. it says:
…”And the successors from his[Hussain a.s.] offspring after the Qaim(Riser) and his Gaiba..”
so who are the successors after the Qaim’s Gaiba? None other than the Mahdis mentioned in the Prophet’s will starting from Ahmed and ending with the last Mahdi a.s.
