Complex sentence: He was responsible for the assassination of Firuz Shah Suri, the twelve year old son of Islam Shah Suri, in 1554 and ascended the throne as the last sultan of the united empire.
Split into simpler sentences: He was responsible for the assassination of Firuz Shah Suri, the twelve year old son of Islam Shah Suri, in 1554. Then he ascended the throne as the last sultan of the united empire.