Complex sentence: In his capacity of Praetorian prefect Sejanus quickly became a trusted advisor to Tiberius, and by 23 already exerted a considerable influence over the emperor's decisions, who referred to Sejanus as "my partner in my toils".
Split into simpler sentences: In his capacity of Praetorian prefect Sejanus quickly became a trusted advisor to Tiberius. By 23 already exerted a considerable influence over the emperor's decisions, who referred to Sejanus as "my partner in my toils".