Rupert Murdoch is locked in a secret legal battle against three of his children over the future of the family’s media empire, as he moves to preserve it as a conservative political force after his death, according to a sealed court document obtained by The New York Times. Mr. Murdoch, 93, set the drama in motion late last year, when he made a surprise move to change the terms of the Murdochs’ irrevocable family trust to ensure that his eldest son and chosen successor, Lachlan, would remain...