Caesar's will is the will of the Legion. And the west... all beneath the flag of the Great Bear exist to test the strength of the Legion.
Legate Lanius, also known as the Monster of the East, is Caesar's second Legate, living in the Legate's Camp in 2281. He is the final boss of Fallout: New Vegas in three of four endings and one of the main antagonists of the game.
Joshua Graham remarks that he did not know of Lanius during his time as Legate, indicating he was not recruited until some time after the First Battle of Hoover Dam in 2277. Asking Caesar about him reveals that Lanius was a member of a tribe hostile to Caesar's Legion, the Hidebarks, and Lanius was the bloodthirsty champion of that tribe, sometimes ambushing entire groups of legionaries by himself. Caesar's Legion then tracked down the Hidebarks, although when they knew that they wouldn't be able to win against Caesar, they chose to surrender. This act drove Lanius into a bloodthirsty frenzy; killing the Chieftain and 15 others of his own tribe single-handedly before he was knocked unconscious. When Lanius woke up most of his face was torn off and Caesar was standing beside him. He offered Lanius a place in the Legion and a mask forged in the image of the great god Mars especially for him. Lanius accepted Caesar's proposal, but in return he wanted permission to kill the survivors of his old tribe (However, Caesar demanded that he only kill the adult males). Since this time, Lanius has led Caesar's Legion to victory over 19 tribes and has led the conquest of Colorado.
Interestingly enough, there seems to be several spoken versions of Lanius's origins, and he himself appears to be surrounded by myths. Lucius mentions that Lanius was already a legionary by the age of 12, which doesn't fit well with Joshua Graham's comment and Caesar's story about him. Also, Lanius is said to be a brutal savage that hates the Legion, only leading it because of his respect for Caesar. However, he demonstrates both cunning and a certain kind of honor; seemingly has no grudge against the Legion itself, only those disobeying orders and not willing to sacrifice themselves for the Legion. Ulysses states he is never seen without his mask and that his own slaves are struck blind so as not to look upon his face, that he is not even certain it has always been the same person without his mask. These opposing accounts show that Lanius's image and past is more a mythic construct within the Caesar's Legion than a flesh and blood man, with the truth of his past only known by Caesar.
Lanius is not well-liked within the Legion, and it seems that both Caesar and Vulpes Inculta harbor doubts at his ability to lead the Legion. Caesar openly admits that he does not think Lanius is worthy of being called a "great man" and believes he has no "love" for his Legion. Vulpes is more reserved, but criticizes his blunt methods and shows quiet celebration if Caesar is saved, thus preventing Lanius from becoming the new leader of the Legion for the time being. Siri, a Legion slave, describes him as not a man to be crossed - especially if you are a woman.
When spoken to, Lanius is revealed to respect honor above everything. He states that treachery should not be relied on as a tactic, and hopes that the Omertas would die after their plan to hold a coup on the New Vegas Strip failed and they were discovered, as their aid in the Second Battle would have sullied the Legion. Lanius also suspects that the Legion took too much of the Dam easily and that the Republic wouldn't be so lax in its security if the Courier bluffs and states that there is a trap set for the Legion. He also shows a dislike for Vulpes Inculta's methods and he criticizes his ploys as "dishonest." He also makes hints that he believes Vulpes is a talker, not a warrior. This shows great conflict swirling around Lanius as Caesar's successor and adds to the beliefs of Joshua Graham, Marcus and Mr. House that the Legion will tear itself apart in the years after Caesar's death.
Veni, Vidi, Vici: Legate Lanius will be the one who gives the final order to the player, and also the one who congratulates the Courier after the battle.
No Gods, No Masters, All or Nothing, Eureka!: At the battle of Hoover Dam, Lanius can be convinced into surrendering with numerous Barter and Speech checks, respectively 55, 65, 75, 85 and 100. One Speech route will convince him that the vast territory of the NCR is its weakness and that the Legion will not be able to claim and hold it without highly weakening their presence in the East. Another Speech route will convince him that factions within Legion want him removed from power and he should take care of said matter first. The Barter route will convince him that the West is not self-sufficient and too dependent on the NCR and its supply caravans, so it would be extremely difficult to maintain. If a Speech or Barter check is failed during the conversation, it is also possible to take a different Speech angle to bluff and convince him to flee, suggesting that this fight was too easy because a trap lies in wait for him, and that the player character is only telling him this because it would be dishonorable to defeat a foe in such a way.
If the Speech or Barter checks can't be passed, or the wrong dialogue is selected at the near-end or the beginning, he will prepare to engage combat, and the Courier will have a unique good Karma dialogue option that will convince him to fight the player character alone, "honorably," regardless of whether or not they have followers with them.
If the player sides with the Legion and allows Caesar to die because of his brain tumor, Lanius will become the new leader of the Legion. He is far more ruthless than Caesar, and handles several things differently than Caesar would have if victorious. For example, he would have Arcade Gannon crucified rather than keeping him on-hand as a doctor, slaughter the people of the Strip upon arrival at the gates of Vegas, have Chief Hanlon crucified with the other NCR Rangers rather than grant him a swift death out of respect, exterminate the Followers of the Apocalypse rather than give them safe passage out of the Mojave Wasteland, and pursue the Enclave remnants rather than recognize it as a lost cause.
Lanius has become a renowned figure in Caesar's Legion sometime after the First Battle of Hoover Dam, since dialogue with Joshua Graham shows he doesn't know Lanius but he has heard of him after his execution. He is also the only member of the Legion to call Joshua Graham by his true name.
When the player asks Caesar about the story of Lanius, he will say that his tribe critically damaged his face. However, when his helmet is removed, his face has no scars or wounds.
In a report by Mr. New Vegas, Legate Lanius practices a version of punishment from the Roman Army called "decimatio," killing every tenth man if they were to lose a battle.
Lanius is the tallest human non-player character in all of Fallout: New Vegas, making him over seven feet tall. (His size is equivalent to setscale 1.15)
He appears to have unique facial hair, with a muttonstache and a goatee.
Lanius performs human sacrifices to Mars, the Roman God of War.
If you try to use any exploits to reach his camp earlier in the game, he will not appear. He only appears at the end of the game. However, he can often be found in the normally inaccessible Legate's war tent.
The final battle with Legate Lanius is more heavily scripted than almost any other encounter in the game. Lanius takes a number of scripted actions during the battle intended to make him a more challenging fight:
When Lanius's health drops below 60%, he will use his healing powder, causing him to regenerate 8 hit points every second for 27 seconds. He has 5 doses of healing powder, and will constantly use them as their effects wear off.
When Lanius's health drops below 40%, he will ironically run off and hide to regenerate health. While doing this, he temporarily becomes non-hostile, causing allies to stop targeting him. This also causes all his limbs to be restored to 100% condition. After several seconds, he will run to his original starting position, and then resume attacking the Courier.
Lanius is 80% faster than a normal non-player character, and is able to outrun the player character even with both of his legs crippled.
Lanius has significantly more fatigue points than a normal non-player character, making him extremely difficult to knock down.
Lanius cannot be disarmed of his weapon.
Lanius's unique version of the Blade of the East has an extremely high knockdown chance, causing him to frequently knock opponents to the ground.