Complex sentence: She runs into Valentine, who ties her down in order to complete his summoning of the Angel by using her blood; however, when Jace arrives to rescue her, Valentine forces her to watch as he fatally stabs Jace in the chest.
Split into simpler sentences: She runs into Valentine, who ties her down in order to complete his summoning of the Angel by using her blood. Then, when Jace arrives to rescue her, Valentine forces her to watch as he fatally stabs Jace in the chest.