During the final battle between you, serena and Harkon (when you choose not to give him Auriel's bow) even if Harkon dies either a. serena keeps attacking his corpse and his health bar starts to replenish or b. he comes back and starts to fight you and serena, but you are unable to do any damage. This has happened to me three times so far so I don't think it's just a hiccup in my game. Thanks for any responses!