From Lancet's excellent walkthtough (http://www.gamefaqs.com/pc/998546-divinity-ii-the-dragon-knight-saga/faqs/62264)
Quote
Path (Moss Rat):
-Return to the first floor of the academy, then put on the HOS and look to the
west. What was previously a walled-in doorway is now open! Head down into the
dungeon.
-Take the HOS off and head across the planks along the north side of the chasm
in the center of the room. When you reach the locked door, turn around.
-Put the HOS on, jump on the glowing platform, then onto a piece of rock on
the north wall.
-Take the HOS off, and jump along the platforms that are only visible without
the helmet. Jump to another piece of rock on the north wall after the third
platform.
-Put the HOS on once more, and jump across the glowing platforms to a ledge on
the south wall.
-Take the HOS off, and jump up the two visible platforms. Claim the Moss Rat
from the cage. (get Moss Rat)