The Sorrow is an ancient enemy of the Changing God. As a result, it is hunting not just her nemesis, but also their discarded bodies, including the Last Castoff.
Little is known about the Sorrow, except that it's ancient, it doesn't pay much attention to established laws of space-time and is intimately tied to the Tides. It appears to have been some kind of guardian against Tidal abuse and evidence suggests it may have been solely responsible for the destruction of one of the prior worlds and the Dalad civilization, which excelled in technologies that manipulated the Tides. The Sorrow descended upon them and wiped them out, laying dormant for centuries, until The Changing God's experimentation with consciousness transfer to save his daughter, Miika, awoke it.
It has been trying to destroy the Changing God ever since, consuming their castoffs in a seemingly never-ending game of cat and mouse.