Get the Anchor Blast upgrade for the Anchor Discharge in the Shattered Library, then head back to the now half-submerged Deep Roads section via the respective eluvian. Use the upgraded ability near a collapsed entrance surrounded by casks with gaatlok to clear the way and enter the mirror. You will find yourself in the hidden section of the Deep Roads called "Unknown Ruin".
Solve the puzzle ("Where the Dread Wolf's gaze blazes, paths are brought to light"). Find the line of tiles leading from the statue to the mural. Put one character on the tile closest to the wolf and tell them to Hold Position. Use another character to press the button on the wolf statue. The tiles will move up in the air, forming a path. Walk up the stairway it to light the veilfire. This yields the staff from the elven box in front of the Dread Wolf statue.