Veni, Vidi, Vici

Veni, Vidi, Vici is an achievement/trophy and final main quest in Fallout: New Vegas if you choose to side with Caesar's Legion.

Detailed walkthrough
First, to initiate the quest, the player must have finished Arizona Killer and by then have access to start "Veni, Vidi, Vici" (which is a final test that results in the Legion attaining victory and control of the dam and New Vegas with the rest of the Mojave). You will start off in Legate's camp, and must make your way to the Legate's war tent where Lanius will open conversation with the Courier, before the attack on the dam. After the conversation with Lanius, the Courier will be summoned outside the tent where he will witness dozens of legionaries preparing to attack and then march towards Hoover Dam. The Courier must then follow the path of the legionaries, past the gate from the Legate's camp to the dam.

The player must fight their way along Hoover Dam to the entrance to the visitor center. Along the way, if the player completed the quests For Auld Lang Syne and Volare!, the Enclave remnants and Boomers will arrive to assist the player. NCR snipers are positioned atop inaccessible towers and must be killed from a distance. Near the visitor center, a handful of Great Khans will assist the player and the Legion if the player did not break their alliance with the Legion in the quest Oh My Papa. At the visitors center the player must enter the Hoover Dam power plant 01 area and fight their way past NCR's forces to the 04 power plant, where the entrance to Oliver's compound can be found.

When the player enters the compound, General Oliver immediately initiates dialogue. With a Speech check of 100, the player can convince him to retreat for the sake of his men. All other options result in a fight. Oliver and the NCR Veteran Rangers with him flee to the second floor as several rank-and-file NCR troopers attack the player. The player must navigate the floor, which includes several force fields that cannot be passed, to reach the balcony where the stairs to the second floor are. Here the player must defeat the two Veteran Rangers, and the floor is heavily booby trapped with numerous grenade bouquets and bear traps, among other dangers. At the end of the room the player will find the stairs to the third floor, where Oliver ambushes the player with several NCR heavy troopers.

When Oliver and all the NCR soldiers in the compound are dead, the player is given the objective to report to Lanius. He will appear before the player upon leaving the compound, congratulating the player on seizing the dam for the Legion and telling them they will be rewarded for winning the Mojave. The ending then plays.

Behind the scenes

 * "Veni, vidi, vici" is Latin for "I came, I saw, I conquered," and was Julius Caesar's description of the short Battle of Zela that he fought against Pharnaces II of Pontus.
 * "In hoc signo vinces" is, likewise, Latin for "In this sign, you will conquer." It was most famously uttered by Emperor Constantine, the first Christian Emperor, who saw the sign of the Cross in the sun and uttered the phrase. By splicing in "taurus," Caesar is exhorting you to conquer for the symbol of the Bull.
 * If Arcade Gannon is following as a companion, he will leave upon arriving at the Legate's camp. Attempting to talk to him again will result in him saying "Why don't you make like Odysseus and get lost?" This is a reference to Homer's The Odyssey in which Odysseus (Ulysses) treks 10 years across the Aegean Sea, trying to return to his home island of Ithaca following the Trojan War, but gets lost and side-tracked numerous times along the way.

Bugs

 * After leaving the Legate's tent, the gate to the dam will not open and you are unable to fast travel thus you are unable to complete the Veni Vidi Vinci quest and finish the game.
 * When entering the Hoover Dam area with Rex and Cass or Rex and Lily as companions, both automatically go unconscious.
 * Raul randomly dies when attempting to enter Hoover Dam from the Legate's Camp. Waiting for an hour as soon as you walk through the door may prevent this.
 * The terminal to release the Legion Centurions inside the Hoover Dam Intake Tower 01 may not appear.
 * If you choose to wait before entering the legate's camp, you might not be brought to the camp when you finally choose to go. Instead it will simply bring up the prompt, and then not do anything after choosing "yes".
 * After talking to Legate Lanius, the screen will fade to black, but instead of the player exiting Lanius' war tent, the player remains inside, unable to move or interact with the world, making it impossible to complete the quest.
 * During the confrontation with General Oliver, after killing him and all his guards the quest will not move on to tell you to exit the room. Despite searching every area for any lingering enemies no red tick(s) will appear. Simply load a save from before the conversation with General Oliver and try again.
 * After killing General Oliver and the rest of his troops, the game will prompt you to exit through the first floor. The game may freeze (leaving you inside the room) when activating the door.
 * During the confrontation with Oliver and his troops the player must navigate through a room full of mines, tripwires, and other traps. If the player quicksaves and quickloads all the mines previously detonated will detonate again.
 * It is possible for an NCR ranger to become friendly in the office trap area. They will flee from the player, making the objective of killing all NCR soldiers much more difficult.
 * Entering V.A.T.S. in the office trap room will freeze the game, no way to fix this, however it only occurs prior to fighting Oliver.

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