Complex sentence: In response, Macrinus sent a cavalry force led by Ulpinus Julianus to regain control of the rebels, however, the cavalry killed Ulpinus and joined Elagabalus.
Split into simpler sentences: In response, Macrinus sent a cavalry force led under the command of Ulpinus Julianus to regain control of the rebels. Instead, the cavalry killed Ulpinus and joined Elagabalus.