Simplify the following text for a Grade 15 native English-speaking person.
Original: During the 20s , Sejanus gradually accumulated power by consolidating his influence over Tiberius and eliminating potential political opponents , including the emperor 's son , Drusus Julius Caesar .
Simplified: During the 20s , Sejanus gradually accumulated power by influencing Tiberius and eliminating potential political opponents , including the emperor 's son , Drusus .