Complex sentence: Not to remain idle however, Macrinus sent Ulpinus Julianus with a force of cavalry to regain control of the rebels, a failed attempt which resulted in the death of Ulpinus and further strengthened Elagabalus' army.
Split into simpler sentences: Not to remain idle however, Macrinus sent Ulpinus Julianus with a force of cavalry to regain control of the rebels. This attempt to bring about an end to the rebellion failed and resulted in the death of Ulpinus and further strengthened Elagabalus' army.