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.

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.

The Aga Khan, who became the spiritual leader of the world’s millions of Ismaili Muslims at age 20 as a Harvard undergraduate and poured a material empire built on billions of dollars in tithes into building homes,