Speak with Manscorpion Tark and he will tell you how he had a master, one that was different than all his kind and coveted that which he couldn't have. This fits Seath, as he was born without the stone scales of immortality that other dragons had. He also mentions that he and Scorpioness Najka were created by this pale one. This also fits with Seath, as he was known to be crazed later in life, running all sorts of experiments on creatures. Upon killing Freja, Manscorpion Tark will utter a new line, congratulating you on killing his master. Although this implies that Freja was his master, it is important to note the spiders full name, The Dukes Dear Freja. This means that the Duke would of likely been in possession of Seaths soul, before dying and Freja ending up with it. We should also note that it is highly unlikely Tark is referring to the Duke as his master.