Complex sentence: After Caster is killed by Archer, Rin Tohsaka then forges a contract with her, becoming her new Master; however, despite being under Rin's command, she pledges her loyalty only to Shirō.
Split into simpler sentences: After Caster is killed by Archer, Rin Tohsaka then forges a contract with her, becoming her new Master. Despite being under Rin's command, she pledges her loyalty only to Shirō.