Complex sentence: Eragon personally battles Durza again, and succeeds in stabbing him in the heart with Zar'roc, but Durza slashes a deep cut like Murtagh's on his back.
Split into simpler sentences: Eragon personally battles Durza again, and succeeds in stabbing him in the heart with Zar'roc, but at a price. Durza manages to slice a deep cut like Murtagh's on his back.