Complex sentence: After receiving his sister's letter, Robb (Richard Madden) decides that Cersei is manipulating Sansa, so he calls his family's bannermen to go to war with the Lannisters, much to Theon Greyjoy's (Alfie Allen) pleasure.
Split into simpler sentences: After receiving his sister's letter, Robb (Richard Madden) decides that Cersei is manipulating Sansa. He calls his family's bannermen to war against the Lannisters, much to Theon Greyjoy's (Alfie Allen) pleasure.