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, to pick up the mininukes, or to find weapons, to 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 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. 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. You should also be aware that using V.A.T.S. to kill the preacher has proven to be unreliable in this situation, as it will likely cause your character to simply shoot the wall instead of the preacher (unless you are directly facing him).

You can also talk the hostage into confronting the Mad Preacher using Charisma, 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 for this.

Reward

 * 5 Mini Nukes
 * 5 Frag Mines