Veni, Vidi, Vici

Veni, Vidi, Vici is the final main quest in Fallout: New Vegas if the Courier chooses to side with Caesar's Legion. Completing it unlocks the achievement of the same name.

Detailed walkthrough
First, to initiate the quest, the player character 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. The Courier will start off in Legate's Camp, and must make their way to the Legate's war tent where Lanius will open conversation with them, before the attack on the dam. After the conversation with Lanius, the Courier will be summoned outside the tent where they 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 character must fight their way along Hoover Dam to the entrance to the visitor center. Along the way, if the player character completed the quests For Auld Lang Syne and Volare!, the Enclave Remnants and Boomers will arrive to assist the player character. 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 character did not break their alliance with the Legion in the quest Oh My Papa. At the visitors center, the player character 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 character enters the compound, General Oliver immediately initiates dialogue. With a Speech check of 100, the player character 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 character 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 character 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, one will find the stairs to the third floor, where Oliver ambushes the player character with several NCR heavy troopers.

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

Behind the scenes
"Veni, vidi, vici" is Latin for "I came, I saw, I conquered" and was written Julius Caesar in a letter to the Roman Senate.

Bugs

 * After leaving the Legate's tent, the gate to the dam will not open and the player character will be unable to fast travel thus they are unable to complete the Veni Vidi Vici 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 one walks through the door may prevent this.
 * The terminal to release the Legion Centurions inside the Hoover Dam Intake Tower 01 may not appear.
 * If the player character chooses to wait before entering the Legate's camp, they might not be brought to the camp when they 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 character exiting Lanius' war tent, they remain 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 the player character 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 one 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 character must navigate through a room full of mines, tripwires, and other traps. If the player quicksaves and quick loads 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 character, making the objective of killing all NCR soldiers much more difficult.