Complex sentence: When al-Mukhtar announced the revenge of Imam al-Husayn, he showed himself as the representative of Muhammad ibn al-Hanafiyyah, the Promised Mehdi according to him.
Split into simpler sentences: When al-Mukhtar announced the revenge of Imam al-Husayn. He showed himself as the representative of Muhammad ibn al-Hanafiyyah, the Promised Mehdi according to him.