At an unknown time the immortal leader of the League of Shadows Ra's al Ghul learned of a prophecy, that foretold he would find a true heir in the form of a young individual who would rise to become the protector & dark hero of the new Gotham City which would be crime free.
Ra's ultimately concluded that the individual was teenage orphan-billionaire Bruce Wayne, this was due to Bruce continuosly impeding the efforts of the Court of Owls who had murdered his parent. Eventually Ra's ordered his agent the Sensei, the leader of the Court of Owls to have Bruce kidnapped and replaced with his clone 514A. Ra's had Bruce taken to the Shamans temple were he was trained in martial arts and mentally conditioned via mind control methods.
Ra's later met Bruce and had him kill Alfred Pennyworth to take the next step of becoming his heir, this was in fact the very step that led to Bruce freeing himself of R'as' mind control. Ra's was both impressed & delighted by this seemingly impossible feat due to it having somewhat proven that he had found his successor. According to Ra's, Bruces' sheer strength and willpower alone is proof enough that he is the one spoken of in the prophecy whom he claims will serve as his "knight in the darkness".