Once you have despatched Voengoethe and his friends, detrapped the boxes/coffins and emptied all the goodies, how do you open the final box/coffin?

One of them says "This will only open for a special key" or something similar. I haven't found a key. I have tried Knock. but that didn't work

Any ideas? What's in there?

Thanks

SPOILERS.....
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You can only open that door after you have advanced a lot further into the plot. It is used to enter Balthazar's Stronghold.

Is there any advantage (hidden items for example) to going thru the "back door" instead of using the direct approach?

No fighting required. ;)