Lucius is the commander of the praetorian guard in Caesar's Legion. He became a member of the contingent in 2268 and became their leader in 2276. In 2277, he took part in the First Battle of Hoover Dam under the command of the legate, Joshua Graham, and therefore is one of the few who know who the Burned Man really is. During the battle, he found himself in Boulder City but he managed to survive the trap of 1st Recon snipers set by the NCR. In 2281, if Benny is captured by Caesar's Legion, it's he who tortures Benny to learn the purpose of the platinum chip.
Despite his age, Lucius has been able to maintain his position in the praetorian guard for thirteen years as no one has challenged him in a battle to the death to take his place at the head of the guard. Caesar suspects this may be more out of his subordinates' respect for Lucius than a result of his strength.
"Caesar has taught us that over-reliance on firearms can only weaken us in the long run. It's why we train heavily with our blades and our fists. Unlike an NCR trooper, a Legionary is always ready to fight regardless of the circumstance he finds himself in."
↑The Courier: "How long has Lucius been the head of your Praetorians?" Caesar: "Lucius has been the head of my guard for five years now. He was a subordinate guard for eight before that. No invitee has dared to challenge him yet. Maybe it's an issue of respect - he is getting on in years." (Caesar's dialogue)