- Get the quest from Zeke in Vault 15.
- Find out where Darion is located.
- Travel to the vault and find him.
- Finish him off in any manner you seem fit.
- Go back to Zeke at Vault 15 for 6000 XP.
In order to start the quest you must have completed the Rescue Chrissy quest. After this is done, go to Zeke, the leader of the squatters, who will give you this quest and the red keycard that gives access the elevator to Vault 15.
If you did not kill Phil when rescuing Chrissy, he will be waiting outside the elevator, where he will initiate dialogue and attack. After he is dealt with, you can enter Vault 15 via the elevator.
In order to get to Darion, you will need to pass a few Khan guards. You can kill them or bypass them peacefully with a high enough speech skill. Darion himself is located on the 3rd level of the Vault near the big computer. Upon seeing the Chosen One, Darion will initiate a short bit of dialogue, after which he will alert his bodyguards and guard dog, equip a flamer, and attack you. You only have to kill Darion in the fight that ensures, but his backup can be hard to avoid.
After killing Darion, go to the first floor of the vault. The present Khans are now alerted to your presence, and will attack on sight, and can be hard to sneak past.
Head for the nearest exit of the vault, and return to Zeke.
You will be rewarded with a generous amount of experience, and can now take the quest; Complete Deal with NCR.