Complex sentence: Robert, Ned and Jon Arryn then rise in revolt, with Ned securing the support of the Riverlords by marrying Catelyn Tully, the daughter of Lord Hoster Tully.
Split into simpler sentences: Robert, Ned, and Jon Arryn then rise in revolt. Ned secures the support of the Riverlords by marrying Catelyn Tully, the daughter of Lord Hoster Tully.