Editor's Note: This is the first installment of a three-part series on the future challenges to the Saudi royal family's rule. The key pillars upon which the House of Saud rests — its unity and leadership, its relationship with the religious establishment and the legitimacy derived from that, and its reliance on energy revenues — will all be called into question in the coming decades. The first real test of the Saudi transition is fast approaching. The deaths of two crown princes in the past eight months highlight the fact that the pool of second-generation princes, who have ruled the country for much of its 80-year history, is dwindling. As third-generation princes step into leadership roles over the next several decades, family unity — a pillar of the family's rule that has allowed it to weather a variety of crises and challenges — will be uncertain.