I don't know as I can give a huge hint without totally giving it away, but I will try my best. I will also compile all the hints that I've given so far.
House of Stewards
Bag End
Osgiliath
House of Stewards
Thingol's treasury
It has to do with people being killed. It does not have to do with who died there. It does not have to do with royalty, money, the location of the building/city, or a reluctance to give up something. It does not have to do with who did the killing. It does not have to do with what the weapon of death was. It does have to do with where the deaths occurred. It does not have to do with when the deaths occurred. It does not have to do with why these people were slain (yes, they were all slain. That means Denethor is not involved here.) You will need to go back to the Appendices, the Silmarillion, and Return of the King to find these particular events. Nilpaurion has come the closest so far.
Other than the above (HINT: reporters should have an easy time of it), the best hint I can give you is to go back and read the whole thread. Also, think about what one place is that the other four aren't. When you figure this out, think about what that one place must
lack as a result of this that the others have.
An event did not take place at the OPO that occurred at all the others.
Nilpaurion, if you've been poked and turn around and poke someone else, that's like withholding chocolate from someone if you've been deprived yourself!