Sir Dante gives the quest to steal this deed from Sir Patrick. Did you get this quest from him in the 1st place?
Procedure:
Commander Ralph tells you something about the murdered Duke and suggests, you should visit Sir Dante.
Sir Dante is willing to give you info if you steal the deed from his rival Patrick first.
What are you missing? Maybe the dialogué with Commander Ralph? He's south of Castle Stormfist in his house.