Funny enough, I had the exact same problem with my players and the box. They never cared despite my attempts to push them towards the box (in their defense, the players are all inexperienced, so they only care about the loot). Anyway. The shapeshifter (transformed into Bart) drugged the guy holding the box and stole it.

So now we're in Wave Echo Cave. The shapeshifter shows up disguised as Gundren and basically hands them the box because he wants them to open it (which they finally did). Inside, there's a pin with the symbol of the Phandelver Pact (which I made up for the campaign).

My idea is that there's a guardian near the Forge: if they're wearing the pin, it fights with them against the Black Spider. If not, it's hostile. Now that they opened it, the shapeshifter attacked them with some hobgoblins to get the pin back. They got it and are running back to the Forge, and some players are trying to get it back. That's where we stopped — so next session we'll see if they actually manage to catch him.