Prince Karim, 88, was given the title of “His Highness” by Queen Elizabeth in July 1957, two weeks after his grandfather, the Aga Khan III, unexpectedly made him heir to the family’s 1,300-year dynasty as leader of the Ismaili Muslim sect.

Hussaini, son of Prince Karim Al-Hussaini Aga Khan IV, has been named in his father’s will as his successor to lead the Nizari Ismailis, a Shia Muslim denomination. We take a look at this branch of Shia Islam that has 15 million members worldwide.

Rahim Al-Hussaini was named Wednesday as the new Aga Khan, spiritual leader of the world's millions of Ismaili Muslims. The 53-year-old succeeds his father, who died on Tuesday.