The Black Rose is a personification of the flower, representing the dark, thorny side of beauty. Love scorned, she is now all about destruction. She is a contrast to the Blue Rose.
(The Black Rose and Blue Rose were originally designed as a 2-sided card.)
More powerful when hired, this card acts similar to the Conqueror, but she may destroy 2 cards instead of kidnapping them.
With her hired bonus, you can do serious damage to any village without protection. She's very destructive, allowing you to send both the defending unit and one other card to the graveyard as her battle reward.
The Blue Rose is good against her, though, so watch out.