Complex sentence: Idris Elba was in talks to play a Merlin-esque figure who trains and mentors Arthur, when Elba did not sign on to the film, the director continued to look for an actor to play the role.
Split into simpler sentences: Idris Elba was in talks to play a Merlin-esque figure who trains and mentors Arthur. When Elba did not sign on to the film, the director continued to look for an actor to play the role.