Fallout 3 You Gotta Shoot 'Em In The Head Walkthrough (Side Quest) - Guide by Carl
This quest can lead you to keys that you need to get to the T-51b Power Armor. In this walkthrough, I'll not only tell you how to finish the quest, but how to get the T-51b Power Armor.
Underworld can be found from the D.C. Mall. If you've been there, this will be very simple to find as it's straight off the mall that leads to the Washington Monument. If not, you might consider doing a piece of the main quest to find the Mall. Look under the Museum of History to find Underworld itself. Crowley can be found in the Ninth Circle at certain times of day. Do some waiting if you don't see him there.
When he arrives at last, speak to him and appeal to his sense of humor to get him to lighten up. He'll give you this quest, along with a Sniper Rifle and a list of ghoul-haters. Each must be killed by a head shot. I'm afraid if you took the Bloody Mess Perk this might be impossible for you because at times the arms and legs are blown up. In this case, you'll only get 25 caps instead of 100 for each kill. Also, if you've killed anyone on his list before, you'll need to turn in their special key to him there on the spot.
Once you've got the list, you can set out to do your contract killings. You'll need to use the Sniper Rifle to do the kills, so save it for these circumstances rather than using it all the time. It's really powerful, so I can see how it could be tempting especially for a Small Guns user.
The Contracts can be found at the following locations, with the following possible outcomes:
Once you have all the keys, you'll have a choice to turn them all in to Mr. Crowley then kill him, or let him go his way. Killing him is preferable. Follow him after he's been given all the keys, but don't ever go ahead of him through a door. He'll eventually go outside and down into the subway. As soon as you enter, you will be out of sight and in a safe place to commit the murder. Take all the keys and you'll have the option to now head to Fort Constantine to get the T-51b Power Armor, one of the most powerful armors in the game.
Fort Constantine is located four squares east and one square south from the northwestern corner of the map. Head into the CO Quarters, which is a small house. Find the stairs down to the cellar (grab the big guns bobblehead in the cellar, too!). The keys will come in handy now, as you proceed to lower and lower levels of the Fort. At the bottom is a room with the T-51b power armor behind a stasis field. Activate the terminal here to get the armor. From there, you can head back out.
Unfortunately, there are no known armors that can repair the T-51b, so you must repair it using a shopkeeper. This can be quite expensive. But then, at the end of the game, you usually have too many caps to spend, anyway!