so heres the thing with the key: 

Its a warrior quest,  but also.  The chest looks like its not an active item when u mouse over it.

So, unlike most keys, where u have the keep in your inventory and then u clik on the box....

you have to right click on the key first.  THEN, when you mouse over the box it says "deadtooth's strongbox" after that u can open it.  Funny thing is. I've been carrying this key all the way up to lvl 45 : )

I've always been meaning to find out what was in the box.  Well, just now, when i opened it, it was empty : )  So maybe u have to have the quest which maybe i abandoned a while back.  Haggard wouldn't give it back to me.  so.... 

there u go...  for what its worth : )