Complex sentence: One night during their training, Ser Brynden Tully quietly killed the sentries, and King Robb's army took the host by surprise, winning Robb an easy victory and badly bleeding the Lannister host.
Split into simpler sentences: One night during their training, Ser Bryden Tully quietly killed the sentries before King Robb's army took host by surprise. Winning Robb an easy victory and badly bleeding the Lannister host.