This is the key to the Free Haven Sewers. It opens the treasure room in the Sewers under Free Haven. However, since Free Haven is in Enroth, which you can only visit in MM6, it won't do your MM7 team any good at all.I believe you picked the key up in the NWC offices. My vague recollection is you got it from looting a dead NWC employee, but I'm not 100% sure of that.
The key was available in MM6, however some people have had difficulty getting it in that game. I don't know the exact details, because I've never actually had any problem getting the key in MM6. I think there is some limitation on when MM6 can generate the key, and if you miss the timing you have to edit your save-game (with an external editor) in order to get the key. Having the key appear in MM7 is sort of a combination apology/in joke.
