tyciol

I was rather confused about this as well. Apparently it's rather close to where you meet Vorador in the garden. You climb the hill you descended to get to Audron's crypt.

Remember the first time we came here to get the spear that penetrates the statue closest to the fire door which leads to the greenhouse (glass room) that opens up into the garden?

We climbed up on the minitature stonehenge thing and had telekinesis'd the window and flew through it. Then we used the fire blade to light 2 fire things on the fireplace at the end. Then we broke it and there were bars and we turned spectral to phase through them.

Then you go through and fly across to that door where we had moved that statue to the middle of the room and turned the black orbs into white (not that one with the gargoyle motion sensors, that was turning white into black and required invisibility and we got there through the air door where we went to the original entrance to the mansion on the upper left window which requiring crossing the pillars).

So apparently it's in there.