In The Hammer of Darkness, where does Thetis fit in? Is it their son whom Martel refers to as “the demigod she tried to protect”? To what does the leaden gray shield refer?

In classical mythology, Thetis was the mother of Achilles, and the implication in the book is that Martel is his father, since Martel twists time to send them both back to the “wine dark sea.” The leaden gray shield is an allusion to a poem about Thetis by W.H. Auden.