Volumnius accompanied Brutus on his fateful campaign to the East, which culminated in Brutus' suicide after his defeat by the Triumvirs in the Second Battle of Philippi (October 23, 42 BC).
After Brutus and Cassius fled east to Macedonia and were defeated there in the Battle of Philippi by Caesar's imperialist successors (the Second Triumvirate, particularly Mark Antony and Caesar's heir Octavian, the future emperor Augustus), Labienus joined the Parthians.