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 (by March 25, 2162), or all the ghouls in Necropolis will have been killed by the super mutants of the Master. (If the Vault Dweller has killed the Master before this time, the ghouls in Necropolis will be there forever. If not, they will die after 30 days.)

If the Vault Dweller talks to Set, the leader of the Necropolis ghouls, and doesn't tick him off, he will give them the opportunity to work for him. He wants the Vault Dweller to kill the super mutants in the Watershed area. Alternatively, one can just go to the Watershed and kill all the super mutants first, 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 a reward. If the Vault Dweller then asks Set for the information they earned, he will warn them to tread lightly as he could turn hostile for it; in which case they would have to fight all of his ghouls.

Alternatively, if the military base has been destroyed before talking to Harry, it is possible to order him and his super mutants to leave Necropolis peacefully, as long as Charisma and Speech checks are passed.

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

If Garret likes the Vault Dweller:
 * 50 bottle caps
 * One shotgun
 * 40 12 gauge shotgun shell

If Garret really likes the Vault Dweller:
 * 200 bottle caps
 * One shotgun
 * 40 12 gauge shotgun shell
 * Four Nuka-Cola
 * Four flares