From what the captive dredger told you, it seems Thrall has been taken deep into Torghast. The torments he is enduring must be unimaginable.
We need to learn where he is being held before the trail goes cold.
The tower is vast and perilous. I suggest you begin your search in Coldheart Interstitia, Mort'regar, or the Upper Reaches. Bring back any clues you find.
Go carefully, <name>. The Jailer's eye is upon you.