Complex sentence: The Clave was alarmed by the sudden disappearance of Jonathan and Jace and started to search for any signs of them, after two weeks of repeating the same statement about what happened at the rooftop, before Clary left him with Sebastian, alive by Simon's blood.
Split into simpler sentences: The Clave was alarmed by the sudden disappearance of Jonathan and Jace and started to search for any signs of Jace and Jonathan. After two weeks of repeating the same statement about what happened at the rooftop before Clary leaving him with Sebastian alive by Simon's blood.