Board Thread:General Grimm Discussion/@comment-49.182.158.52-20150823013829/@comment-1509826-20150826112332

No one knows what the treasure is, just that it was considered far to powerful for the Royals to ever get their hands upon it. So much so that seven Grimm knight's rebelled, stole it and buried it in the Black forest.

The only book about it, was destroyed in the Wesen Wars, so that only a single page survived (and that probably went up in flames when Juliette burned the trailer).

Back in Season two Renard admits that even the royals have no idea but suggests four possibilities based on popular legends: the secret of immortality, the secret of alchemy, the prophet Muhammed's sword or the nails of Christ's crucifixion along with perhaps pieces of the cross itself (and in a universe where Jewish prayers can create a golem, and native American ones can summon and banish such a terribly powerful spirit. Just imagine what either of them could do).

Likely chance is none of those are going to turn out to be correct, but you can see why they would be deemed to powerful for anyone to control, let alone a power hungry organisation that wants to plunge the world back into the middle ages.