Eureka!

Eureka! is the final main quest in Fallout: New Vegas, if the players choose to side with the NCR. It is also an achievement/trophy.

Synopsis
The Second Battle of Hoover Dam is erupting as Caesar's Legion storms the dam and attempts to snatch it from New California Republic control. The Courier fights alongside NCR forces to make it to the Legion's base of operations and either kill Legate Lanius or force the Legate to surrender.

Walkthrough

 * 1) Get briefed by General Oliver.
 * 2) (Optional) Speak to Mike Lawson and flush the Legion invaders through the turbines. Passing a Science check of 65 allows the Courier to use the terminal on the second floor of Power Plant 03 will engage the dam's turbine and cut off the Legion's approach into Hoover Dam.
 * 3) Make your way through the power plant and across the dam.
 * 4) Arrive at the Legate's camp across the bridge and deal with Legate Lanius.
 * 5) Kill him through conventional means
 * 6) Alternatively, make a series of speech checks that requires 100 in either Speech or Barter to convince him to leave. If one fails the last Speech check, one can still pass a Speech check of 85 to fight him one-on-one.
 * 7) Alternatively, make an easier set of Speech checks (of 60, 75, 90, and 90) and bluff him out by choosing the option after the last Speech check and causing him to flee.
 * 8) Once having dealt with the Legate, speak with General Oliver.

Behind the scenes
"Eureka" is the state motto of California. The word comes from the ancient Greek word, heúrēka, meaning "I found it". There is a story of the famous Greek mathematician Archimedes, who shouted "Eureka!" upon making a great discovery and ran naked through the streets of Syracuse.

Bugs

 * 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 become hostile to you.
 * Even after you kill Lanius, the quest marker will remain on him until all of Caesar's Legion in the area are dead. This can sometimes break the quest as the legion in the area are effectively stored in an unreachable area to the left of the main building were you encounter Lanius. This can be fixed via explosives such as rockets to kill them via blast radius.
 * It is possible for the player to fast travel when this quest is in progress by simply going through the Hoover Dam checkpoint and straight after you come out quickly go to your Pip-Boy and fast travel. After doing this you will have to use that same method if you want to fast travel again.
 * 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 cannot be interacted with.