Well considering almost every item in DS 1 and 2 is a direct Zelda derivative, you could ask this question in another section that shall not be named of this site, but there is one notable exception - the Soul Splitter. Sure, everything is derivative in some way, even the Soul Splitter, but I think it is utilized in some of the most clever situations in the entire series, and stands out with not just splitting yourself, but leaving the statue behind that is where your physical form always has to stay (and sometimes figure out how to move), creating some very cool puzzles.
And, it is hilarious that it breaks many of the combat encounters in the game. Lead to some good laughs throughout the many, many hours I spent in DS2.