The party finds a book, a second copy of a book they have or have seen before, or can look at. On reading and/or close examination they find that the new copy has an extra passage/paragraph detailing where the famous hero/ine was buried/trapped. The book could be a history of the land, a tale of brave deeds etc. No other copies of the book have this passage, wise persons who are familiar witht the work can't recall the passage in anything they've read (but maybe someone will partially confirm the rumor...).
Once they get there there are a few options (in order of time consumption increasing concerning the book):