Eureka!

 is a final quest in Fallout: New Vegas.

Description
Take over the Hoover Dam with the NCR.

Quick walkthrough

 * After completing You'll Know It When It Happens, talk to General Oliver.
 * The player must defeat multiple difficult groups of enemies as they make their way through the power plant and across the dam, aided or unaided by NCR Rangers.
 * On arriving at the Legion camp across the bridge, speak to Legate Lanius
 * By passing a speech or barter check (100), the player can convince the Legate not to fight.
 * Otherwise, the player will have to defeat the Legate
 * Once defeated, speak with the General

Detailed walkthrough
After completing You'll Know It When It Happens, at the very moment you choose to side with the NCR for the final battle (thus completing For the Republic, Part 2, you'll be automatically taken from Col. Moore's office to General Oliver's Compound, where the General will begin briefing you. The player is given the chance to wait, as this begins the end quest for the game. If the player chooses to continue, Legion members attack the Hoover Dam and you fight your way through the power plant rooms and head towards their base east. You will start in Power Plant 04. Mike Lawson will speak to you. If you wish to follow his suggestion and do the optional part which involves flushing the Legion invaders through the turbines, agree to his suggestion. Through the rest of this, kill any Legion troops you may encounter along the way. Enter Power Plant 03.


 * (Optional) To complete the optional part of the quest, follow the staircase immediately to the left leading back upstairs to enter Power Plant 03. The console is on the second floor of Power Plant 03, on the southern wall. Passing a Science check (65) allows the player to use the terminal to engage the turbines and flush the Legion troops out.  Completing this will require manual intervention; return to Power Plant 04. On the east side, go up the stairs in the middle of the room between the two large tanks. On the east wall there is an exit to Hoover Dam. There is a manual release valve along the river to your left between the broken retainer wall, just after the second sandbag wall. Return to Power Plant 03.

Proceed to Power Plant 02, then Power Plant 01. Climb the stairs on your left in Power Plant 01, then proceed north along the balcony. Follow the passage along the left, then go up the stairs, and exit via the elevator to the Hoover Dam Visitor's Center. At this point, you will begin to notice assistance appearing from companions and factions you have helped out along the way. Exit via the double doors on the east side to Hoover Dam.

Proceed to the north along the curved walkway above the dam. Take care along the entire dam not to go too quickly or you will end up with enemies flanking you. Enter the Hoover Dam Checkpoint, then exit on the other side. An NCR Ranger Veteran Commander will thank you for your support, asking how to coordinate their assistance. The player can respond with several dialogue choices:
 * With a sufficient speech skill (85), the player can say "You have superior firepower. You can overwhelm them if you push the advance on them." The NCR will attack the Legion soldiers by themselves, clearing the path to the Legate.
 * "I need some snipers to cover my advance."
 * "Have your unit follow me, we'll assault them head on."
 * "I can manage this without your assistance."

After crossing the dam, head left along the bridge to the east, and continue, following the road. Enter the Gate to The Legate's Camp. Head east into the camp, following the path up the side of the ridge. You will fight Legate. (With a speech skill of 100, you can pass a series of skill checks, and avoid fighting him.) Legate is extremely challenging, not to mention the dozen or so minions that come out of the woodwork. There is a small niche of boxes and scrap to the right of the main stairs path that the Courier can hide behind to allow Stealth Boy sniping.


 * There is another glitched spot in this camp where you can stand unharmed and take everyone out one by one.  Make your way to the top of the hill to the tent building.  Walk between the right side of this building and the Legion flag pole, and no one will be able to reach you in there.   Legate, himself, can only stand helpless in front of you, while you take him out with any weapon of your choice.   As soon as you're able, head for this spot.   Makes the finish so much easier when you can't be hurt, obviously!  Otherwise, it's a nasty battle.

After either killing the Legate, or passing a series of speech checks(requiring level 100 speech) to end the fight peacefully, you should notice the gate to the camp has closed. Head back to it. The gate will explode, you will meet with the General. Enjoy your endgame montage.

Bugs

 * The crazy nightkin that spawns near Brooks Tumbleweed Ranch and tries to sell you tumbleweeds may spawn along the top of Hoover Dam during the battle.


 * If you enlisted the help of the Enclave Remnants sometimes Doc Henry will basically "clone" himself and will appear as himself in an Enclave scientist suit and there will be another Doc Henry who appears as he does when he is in Jacobstown.


 * Occasionally, leaving Power Plant 1 without completing the optional quest marker, you will randomly fail 'Don't Tread on the Bear!' and all NCR forces will go hostile to you. (Confirm, XBOX360 PS3, PC)


 * Sometimes the Legate will not register as dead and you cannot continue with the quest. POSSIBLE Fix: If you go through the speech checks and convince him to fight you one-on-one, he will register as dead and you can continue the quest.


 * There is a bug where your followers (if any are brought) will not follow you unless you tell them to.


 * If you enter VATS and scroll through all targets more than once, it may turn foggy and occasionally lock-up.


 * If you swim across, NCR will turn hostile.


 * If you have ED-E as a companion when you choose to activate the quest, for some reason he will not follow you into Power Plant 4, leaving him behind in the locked room.


 * Using setstage 00136166 100 spawned none of the needed minions to continue the mission regardless of the fact that the player has progressed and done all available side missions and fought through to the legates camp (as the previous mission had bugged).


 * If the final missions have bugged out while trying to help the NCR (as happens if you complete Wild Card: Finishing Bets before For The Republic Part 2), you can use setstage "00136166" 100. You then fight your way to the legate camp but Legate Lanius won't appear. You can then use setstage "00133075" 20 and General Lee will appear as you go to leave the legate camp, thereby starting the endgame sequence. I have tried spawning a Legate Lanius but he is immediately hostile and set to essential so he's not much use.


 * Doors may require key and cannot leave Visitor Center. SOLUTION: apparently if the staging area is left too soon after the president's departure, the doors will not unlock. They appear to unlock shortly before the "Quest Completed" finishes fading from the screen. a safe bet would be to save just after the president finishes the speech, and checking the doors before continuing onto another area.


 * The Legate has been known to flee once the two NCR Rangers appear, he will go out of the main gate, and you cannot follow him as the gate will have no sense of interaction.