Complex sentence: After crossing, he destroys the Lannister army besieging Riverrun; and after hearing of his father's execution, Robb is crowned' King in the North', and invades Tywin's realm of the Westerlands.
Split into simpler sentences: After crossing, he destroys the Lannister army besieging Riverrun. Upon hearing of his father's execution, Robb is crowned' King in the North', and invades Tywin Lannister-controlled realm of the Westerlands.