Get rid of the Nightkin in the basemant.

I killed all the Nightkins and helped Harland out in the basemant.
But I could not get in the underground (no access, glitch).
The quest would not move on. What should I do next?

Who's Harland? I just finished that quest...are you sure you did kill all the Nightkin?

Harland is the ghoul that "got trapped" in the basemant.
I walked through the basemant a few times. I didn't see any Nightkin.

Did you find the named Nightkin?

Yes, I killed Nightkin Davison and the jail watcher.

People were complaining that if you went into the basemant all saved games after that point would cause the game to crash to the desktop. I wonder if they have removed the access to the basemant till they get it patched. I killed all the big visible and invisible monsters (can't remember the names) and Harland and the entrance to the basemant that shows up in the local map on the PipBoy has no access.

Interesting, i am sorry that you are having trouble. I am on PC and did that quest a few days ago with out any problems. It was one of the first few i did as i stumbled my way around the desert trying to avoid joining the en'tres on the menu. I do remember they were a pain in the ass to do as it seemed a bullet to the face just made them angry and i died quite a few times completeing it.

Asai

To open the door you must use the switch on the back wall, although you should be told to talk to Harland again before going down there if your quest didn't update there's still a nightkin hiding somewhere.

I loaded back to an earlier point and started over the quest.
I have just finished the quest. I think I must had missed a nightkin in somewhere.
Thank you all of you for helping.

Well, I'm not going back to find Harland...I did find a dead ghoul in a cell, but creeping round in the dark looking for invisible monsters was a bit more important than ratting a ghoul corpse at that point.