Yahar’gul, Unseen Village
First up, proceed far enough through the story until you have defeated Rom, the Vacuous Spider, after this, head over to the Great Cathedral, careful as there’ll more than likely be a very powerful NPC waiting for you here
Once outside the Great Cathedral, take a left and follow the path all the way down to the Amygdala, be careful through here as there are several very powerful enemies
Once you can see the Amygdala, run straight through, don’t stop, continue through to be in Yahar’gul, Unseen Village, ensure you use the lamp here so if you die have a shorter distance to backtrack
From the lamp, head down the stairs, take a right and follow the path through, then head down, don’t proceed through the doorway, instead, take a left, through the broken section of wall, go to the end of the corridor then take a quick right and go down the stairs, take a left going through the open door, go down the stairs, then back up the stairs opposite, take a right then look for a broken section of railing, carefully drop through this opening then head right, drop off the ledge to the left to land in a cell, just outside this cell on a corpse is the Tiny Tonitrus