The Preacher

 refers to an unmarked quest in Fallout 3.

There is a mad preacher ("Wastelander") standing on the second floor of a building in an alleyway west of the Reilly's Rangers headquarters entrance on Seward Square. The alley has been rigged with mines and nukes that he detonates if you try to get close to him. There is a hostage (another Wastelander) on one end of the alley, who you can talk to. If you are looking at the mad preacher when he detonates, he waves at you. He also dies in the blast.

Resolving the situation
If your stealth is high enough, you can sneak through and pick up the Frag Mines (at least five) and multiple Mini Nukes (there are at least five; one behind and one in between the barrels by the slide, one next to the shopping cart, one on some rocks across from the shopping cart, and one on the merry-go-round.) Even without the Light Step perk, the explosives will not detonate if you step on them as they are intended to explode only when triggered by the preacher. Alternatively you can use Dogmeat to find ammo, pick up the mininukes, or find weapons and pick up the Frag Mines. However removing all the nukes and mines off the ground will still NOT stop him! The Preacher can still cause a deadly explosion if he sees you in the alley. There are at least five more Mini Nuke detonations that can't be stopped.



The only sure way to stop the Preacher and the detonations is by killing him. This can be achieved by getting to a position where you can properly see him and picking him off with a well-placed shot; a good spot is on the street behind the hostage, just move to the left on the street until you can see the preacher. You may even sometimes be able to take the shot from right next to the Wastelander who asks you for help. Be careful though, because if the preacher sees you, he will order you to freeze in your position, and any movements can cause him to trigger the explosives. More importantly though, if you take the shot but somehow miss, he will detonate the bombs. Firing at him with weapons like the Missile Launcher (which take longer to hit him) may cause him to detonate before he dies as well. Tossing a frag grenade into his hiding spot while out of sight is also an option for killing him.

You can also talk the hostage into confronting the Mad Preacher using Charisma (a score of 1 is required), which will cause him to boldly run towards the preacher to talk to him. At this point it would be wise to back away from the alley. You get negative Karma (no karma change on Xbox 360) and a spectacular fireworks show for this.

An alternate method (confirmed on PC) is to throw a frag or plasma grenade into the alley and backpedal. The preacher will be killed by the resulting explosion. This route nets no reward, or karma change.

Reward

 * 5 Mini Nukes
 * 5 Frag Mines

Frag Mine Locations

 * 3 clustered in the center
 * 1 underneath children's slide
 * 1 on broken piece of teeter-totter

Mini Nuke Locations

 * 1 behind three metal barrels
 * 1 in between those barrels
 * 1 on a pile of rocks to the immediate right of the boarded up Preacher's door
 * 1 on far side, on a merry-go-round
 * 1 immediately next to or in the shopping cart (opposite the door)

Trivia

 * The "great fat worm" is likely a reference to the poem The Conquerer Worm by Edgar Allen Poe.
 * He also talks about putting the sun in a glass jar, and "breaking it over ourselves" which could refer to the nuclear weapons used in the Great War