Does anybody has an idea what Grallaks silver key is for?
It was originally put in there, to unlock the door leading to the Labyrinth. However, the powers that be at Dungeon decided that they didn't want to force DM's (or players) to have to run through the dungeon in a certain order. So the text explaining the keys was removed, but not the keys themselves. You can ignore it, or you can say that it unlocks the door to the Laybrinth.
This was a fairly confusing point for me and my group.
I kept searching for information regarding the entrances to the various temples, and couldn't find anything :) But then the keys were mentioned, so the inconsistancy threw me for a loop.
I ended up doing some handwaving and my players realised that these weren't the keys they were looking for and everything was ok...
Luckily for me, none of my players are immune to Jedi mind tricks.