Complex sentence: After Geta's death, the weapon which killed him was dedicated to Serapis by Caracalla, most likely to cast Serapis as Caracalla's protector from treachery.
Split into simpler sentences: After Geta's death, the weapon which killed him was dedicated to Serapis by Caracalla. This was most likely done to cast Serapis into the role of Caracalla's protector from treachery.