For almost 13 centuries, from the death of the Prophet Muhammad in 632 to the overthrow of the last Ottoman caliph in 1924, the Islamic world was ruled by a caliph.

Translated from the Arabic ‘Khalifa’, the word ‘caliph’ means successor or deputy.

The caliph was considered the successor to the Prophet Muhammad.

It is a term that has, at times, been abused.

In June 2014, an armed group calling itself the Islamic State in Iraq and the Levant (known as ISIL or ISIS) declared the establishment of a caliphate and proclaimed its leader, Abu Bakr al-Baghdadi, a caliph. This proclamation was rejected by the overwhelming majority of the world’s Muslims.

ISIL had attempted to appropriate a title imbued with religious and political significance – and in doing so had cast a dark shadow over a rich history.

This is the story of the caliph, a title that originated 1,400 years ago and that spanned one of the greatest empires the world has ever known.