Walk the wasteland long enough and anyone will gather a few scars. Gecko bites, laser burns, scorpion stings. The little scars make for nice talk at parties, but some scars are so deep, so big that they end up defining a person for the rest of their lives. You see a man burned head-to-toe and he keeps on walking, or someone with their face torn off who refuses to lie down and die - That's a person who has more important things to do than tell you their story over a bottle of nuka cola.
When Caesar and his Legion came upon the Hoover Dam in 2277, he knew that it was his destiny to march his army across the Colorado river just as the first Caesar had crossed another river two thousand years before. Unlike the original Caesar, this new Caesar found himself facing a foe that would not yield at the mere sight of an approaching army. The New California Republic was just as determined to hold that dam as the Legion was to take it.
When the battle came, Caesar's legate made several mistakes in the command of his forces. A combination of overzealousness and righteousness made the Legion an easy victim for a trap laid by the NCR, and the resulting debacle put an end to the Legion's march West. Now they wait on the far bank of the Colorado river, rebuilding their army with fresh slaves taken from the fiercest tribes East of the dam, looking for a way to send the NCR running in earnest terror.
Fallout History Caesar and Joshua Graham
The man who led them to their catastrophic defeat had been Caesar's first General. At the time he went by the title Malpais Legate, but he was once known as Joshua Graham, one of Caesar's oldest friends. Like all men, Caesar feared the unknown, and placed his trust in the people he knew best. Malpais Legate might have been the person best qualified to lead the assault and simply fell victim to a cunning ploy by the NCR commanders. Or maybe Caesar was unable to see the limits in his old friend's abilities, and should have led the charge himself. Either way, Malpais was punished for his failure in the worst way the Legion could find. Burned alive and thrown into the Grand Canyon. Even if Graham managed to survive, he'd be disfigured and in agony for the rest of his life.
The next time that the Legion sets out to take that damn, they'll need a new Legate in command of their army. They'll need someone who can stir genuine fear in the hearts of the NCR troops. and they already have the perfect candidate. Joshua Graham ended his career with defeat and disfigurement, but his replacement joined the Legion through the same path.
Radiation, mutation, cybernetics, these can all create monsters, but the one they call The Monster of the East is just a man. He was not created by science, but by rage. Like all those who were assimilated into the Legion, he once went by another name, although that name is lost to history. He was the greatest warrior of the Hidebark tribe to the East. His brute strength and massive size put him on the path of the warrior at a young age and by the time he was an adult he had become the most feared man in his tribe. He was their defender, and the neighboring tribes learned to avoid Hidebark territory.
When the Legion first arrived, they quickly discovered that any soldier who entered Hidebark lands would be slaughtered by a giant swordsman who left behind a trail of bodies cleaved by a massive blade. The Legion named this unseen terror "Lanius" the Latin word for butcher.
There are very few tales of the Hidebarks that do not center around Lanius. A clan of savages whose only remarkable feat is that they survived for two hundred years after the bombs fell. As Lanius grew to manhood, the tribe in turn grew to depend on him too much. With such a terrifying champion to fight their battles for them, the Hidebarks became soft and weak over time. Their chieftains needed neither wisdom, nor might to flourish, so long as Lanius was there to protect them.
When the Legion arrived looking for new tribes to enslave, the chieftain of the Hidebarks feared that this was a threat even Lanius could not defeat, and the tribesmen chose to surrender to Caesar, rather than face death in battle.