Complex sentence: They catch the "Mistmaw" before it can rendevoux with the Auroran fleet and disable it, however before they can affect the surrender they are attacked by the Auroran flagship, the "Itasca".
Split into simpler sentences: They catch the "Mistmaw" before it can rendezvous with the Auroran fleet and disable it. Before they can affect the surrender, they are attacked by the Auroran flagship "Itasca".