Legate Lanius

Lanius is Caesar's Legate, living in the Mojave Wasteland in 2281.

Background
Asking Caesar about the legate reveals that Lanius was a member of a tribe hostile to Caesar's legion and Lanius was a bloodthirsty champion of the tribe, sometimes ambushing whole groups of soldiers by himself. When Caesar's legion was attacking his tribe, they knew that they would not be able to win so they sounded the surrender alarm, driving Lanius insane with anger, resulting in beating the Chieftain to death which caused his tribe to turn hostile to him. After taking out fifteen of his tribesmen, Lanius woke up in a bed with most of his face torn off, Caesar showed him the mask he had forged for him and offered him a place in his army.

Interactions with the player character
At the battle of Hoover Dam, if against the Legion, Lanius can be convinced into surrendering with numerous Barter and Speech checks, respectively 55, 65, 75, 85 and 100. Convincing him that the vast territory of the NCR is its weakness and that the Legion will not be able to claim and hold it.

It is possible to kill him in one shot before he notices you, bypassing the conversation entirely. Before he talks to you he will appear as a friendly, however.

Strategy
One of the many ways to kill Lanius is with the use of a ballistic fist in combination with any chem that can boost your DAM. It's also a good idea to bring a lot of stimpaks as Lanius will not be alone and will be backed by several veteran legionaries and Praetorian Guard. The higher your unarmed skill the faster Lanius will go down.

Another valid strategy is to sneak attack critical headshot Lanius using a fully repaired Gauss rifle (Fallout: New Vegas) with Psycho and maxed out energy weapons skill. This will bring him down to a state of near death, easily finished off with some plasma rifle shots.

Appearances
Lanius appears in Fallout: New Vegas. He serves as the final boss of the game during the final battle at Hoover Dam, unless you side with the legion. He is carrying the Blade of the East.

Behind the Scenes
"Lanius" is Latin for "butcher." And "Legatus" also know as "Legate" are the generals of the Roman Empire, equivalent of the modern general officer.

He constantly mentions Mars, the Roman God of War, and claims that his name (Lanius) comes from the Son of Mars, being Caesar himself.

If you remove his helmet you can see that his face is perfectly intact, despite the story Caesar tells you about how most of his face was torn off.