All or Nothing

All or Nothing is a final main quest in Fallout: New Vegas if you choose to side with Mr. House.

Quick walkthrough
You will spawn at Hoover Dam in the middle of a battle. There will be several factions that you will have to deal with to get to the door, but you will have the help of several Securitrons. After getting inside and installing the override chip, you will need to take a fairly short walk and turn on the power to turn on the Securitron Army. After that, you must exit the Dam and head east towards the Legate's Camp and kill the Legate and all enemies from Caesar's Legion. It is also possible to talk them into leaving the dam, with a Speech challenge (100 Speech) or a Barter challenge (100 Barter). Once you have killed all enemies in the Legate's Camp, you must head back west where you came from and talk to General Oliver and deal with him and the NCR. You may be able to avoid killing Oliver and his men if you pass a Speech challenge (100 Speech), Science challenge (100 Science) or a Barter challenge (100 Barter). After dealing with Oliver, talk to Mr. House and end the quest.

Detailed walkthrough
You will spawn at Hoover Dam where immediately NCR Rangers will be fighting the Great Khans (if you did not make them join you in the quest "Oh My Papa") and Caesar's Legion. The tower you must enter will be the third one on your right. You will see two NCR heavy troopers who will turn hostile if you plant the override, but you can lie to them with a speech check of 75, allowing you to use the override without having any NCR soldier turning hostile, which is easier for your escape. Once you have uploaded the override, head left until you find the door to the main power plant, where just outside there will be the electrical switch to turn on the power to turn on the Securitron Army. Head back in, find the key to the exit door and go back to the dam where only Legion and Securitons remain, and head east towards Legate's camp. Securitrons will be helping you fight your way to the camp.

Once at the Legate's Camp, you must either kill the Legate and all enemies from Caesar's Legion or convince the Legate to retreat with a speech of 100 (some members of Legion wish to usurp him) or a barter of 100 (The West will be impossible to hold). Passing a lower skill check of 80 in either skill will convince Legate to fight you alone, and his bodyguards will remain non-hostile, even if you have a Vilified reputation with Legion.

Once you're done with Legate's Camp, you must head back towards the gate where you must deal with General Lee Oliver and the NCR army. You can either kill Oliver and his men or convince him to surrender the dam with a speech of 100 (House's forces are much stronger than the NCR forces), science of 100 (House can run the dam better than NCR), or a barter of 100 (House agrees to protect Caravan routes). After dealing with Oliver, Mr. House will roll up in a Securitron Mk I, tell you Oliver and Kimball's likely fates, and end the quest and game.

Journal entries
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