In the same filing cabinet as the one holding evidence that it is possible to be born in the wrong body?
It's in the same vault as the Necronomicon
I know this because on my way back from this discovery I tripped over and stumbled into a portal where I found myself at the bottom of a shaft but was collected enough to observe that the slate slab underneath bore the inscription “HIC IACET ARTORIVS REX QVONDAM REXQVE FVTVRVS".
Sadly, however, the head injury that I received in the fall and my Batman Begins style of climb to escape that pit and shaft means that I can not, to my sorrow, recall the location of that vault.