Use the Orb of RegretAn entrance to Tiamat's Hideout, located in the Misty Abyss. to enter Tiamat's HideoutWhere the Balaur Lady Tiamat can be found. You can access it using theOrb of Regret..
Secure victory against the 5th Dragon Lord Tiamat.
Go to the Misty AbyssThis is home to theOrb of Regret that contains the remains of the 5th Balaur Lady Tiamat. and talk to Kasis.
Summary:
Secure victory against the 5th Dragon Lord Tiamat.