Destroy the Super Mutants at the Watershed

Destroy the Super Mutants at the Watershed is a side quest in Fallout.

Detailed walkthrough
The Vault Dweller needs to start this quest before 110 game days have passed, or all the ghouls in Necropolis will have been killed by the super mutants of the Master. (If you have killed the Master before this time, the ghouls in Necropolis will be there forever.)

If you talk to Set, the leader of the Necropolis ghouls, and manage not to anger him, he will give you the opportunity to work for him. He wants you to kill the super mutants in the watershed area. Alternatively, you can just go to the Watershed and kill all the super mutants first and then ask Set for payment afterward. Go to the watershed through the sewers, and prepare for a difficult fight.

There are a total of six super mutants in the area, including Harry, who is the most heavily armed. Once all are dead, return to Set in the evening to receive your (frankly) paltry reward. If the Vault Dweller then asks Set for the information you earned, he warns to tread lightly, as he could turn on you for it and you'd have to fight all of his ghouls.

Alternatively, if you have destroyed the military base before you talk to Harry, it is possible to order him and his super mutants to leave Necropolis peacefully, as long as you pass a Charisma and Speech check.

Rewards
Normal:
 * 40 bottle caps
 * 4 Nuka-Cola
 * 4 flares

If Garret likes you:
 * 50 bottle caps
 * 1 shotgun
 * 40 12 gauge shotgun shell

If Garret really likes you:
 * 200 bottle caps
 * 1 shotgun
 * 40 12 gauge shotgun shell
 * 40 bottle caps
 * 4 Nuka-Cola
 * 4 flares