The Preacher

 refers to an unmarked quest in Fallout 3.

There is a Wastelander (commonly known as "The Preacher") 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 Mini 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 Preacher when he detonates, he waves at you. He also dies in the blast.

Resolving the situation
There are a few different routes that can be taken to achieve a resolution. You can talk to the guy in the alley and have him confront the preacher. Doing so will result in the detonation of the mines and mini-nukes. This will result in the preacher's death as will resolve the situation. Alternatively, the PC can kill the preacher using a ranged weapon of choice.

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 the far side of the alley, on a merry-go-round
 * 1 immediately next to or in the shopping cart (opposite the door)

Quotes
General
 * "And the sun will rise in the north! No! the northwest! For forty-four days and sixty-seven nights! And we'll never notice!"
 * "You don't know, you can't know. Little fools. You wish you knew! You wish! But only I know! Only me... only me... only me..."
 * "The sky will fall! The Earth will rise! The stars will shine open their toothy grins and caress the sun with their love!"
 * "THE SUN! IT BURNS! Why does it burn? Why have we put the sun into a jar? And what did we do with that jar? We broke it all over our little world..."
 * "Fire, fire, burning bright... burning... fire... pain, misery, peace, love, murder, death, death, death, death...! love."
 * "Trees! So many trees! To the north! But be careful... the trees... are deadly, and the trees too will be consumed in the belly of the great fat worm!"
 * "A great crack shall open in the Earth and swallow the non-believers! And they shall weep! Weep! Weep! Tears and salt and earth and dirt!"
 * "Try new BLAMMO! Soap! Blasts stains out! Blasts the whole world out! We've all been BLAMMO'd!"
 * "In fourteen hundred and ninety-two, your mother and my mother were hanging up clothes around the mulberry bush..."
 * "Oh... God... it hurts. My head... why doesn't it ever stop hurting... why? Please... please I can hear it! Moving! Pulsing! Just leave me alone!
 * "Not one step further! Not one! Come no closer! I'll do it! I'll blow us ALL to hell! All of us! Me, you... and the worm..."
 * "Beware, worm! I shall destroy you! With the fire of this small sun before me! BEWARE WORM! Not one step further!"
 * "Hungry... hungry... so hungry. What? No. I'm not hungry. The worm is hungry. Hungry for me, for you, and for fire."

Trivia

 * The "great fat worm" is possibly a reference to the poem The Conqueror Worm by Edgar Allen Poe, or Shai-Hulud, a giant worm featured in the Dune book series by Frank Herbert.
 * The "great fat worm" is possibly a reference to Níðhöggr "wyrm" that devours the roots of the world tree in norse mythology, partly to do with the mention of the Oasis being devoured by the "great fat worm."
 * He also talks about putting the sun in a glass jar, and "breaking it over our little world," which could refer to the nuclear weapons used in the Great War, especially since he threatens to kill the player with the "small sun before [him]."