This is the final quest of the game if you picked the Legion side. The game does not continue after it, so be sure to keep a save file from before. It is also a good idea to get a lot of Stimpaks and Med-X and bring all your best weapons.
You will be asked if you are ready for the final fight, and once 'yes' is picked, you are transported to the Legate's Camp. After talking with him some, you have to storm the Hoover Dam with the mission of killing General Oliver.
There are four areas of the Dam to be fought through, with the factions you have befriended helping you throughout. There are Legion waiting in the intake tower who will help once released, too.
Once you have found General Oliver, if you have speech of 100 you will be able to convince him to stand down, otherwise, prepare yourself for a long and difficult fight.
The final area is filled with traps and NCR Veteran Agents which can hurt if you are not on your toes. Going up the stairs takes you to the last room where General Oliver is waiting with 6 Heavy NCR Troopers. It is best to use some Med-X here as you will take less damage, and top up when needed. This will be a difficult fight as they do a lot of damage and are heavy armored, so using armor-piercing rounds is advised. Keep your Stimpaks on a hotkey and keep laying into them and eventually they will go down.
Once General Oliver is dead, leaving the room will have the Legate congratulate you for your contribution, and the game will end.