Rumiko Takahashi is one of the most famous and renowned manga authors ever to put pen to paper. Her work on Urusei Yatsura and Ranma 1/2 have made her internationally famous, but she's had other hits too. I'm not sure why but I've never been too interested in the latter, (You're missing out - Ed) but Urusei Yatsura is one of my favourites, and so is this.
Unlike Takahashi's other major series, Mermaid Saga can be very dark and very gritty, not shying away from death, murder and betrayel. It follows the adventures of two young people, Yuta and Mana, who ate the flesh of a mermaid. Mermaid's flesh is legendary, it is general knowledge that eating it can grant eternal youth and immortality, however, it can also be a lethal poison, transforming the eater into a hideous, uncontrollable beast before dying. Luckily, our two heroes acquired immortality, but is eternal life all its cracked up to be?
The series follows the pair as they encounter others who have partaken of the sin of eating mermaid flesh, and such encounters are often sinister. It is this grittiness and darkness that makes the series so damn good in my opinion. In terms of genre I put horror because, at many times, if the story was a movie, it would definitely be a horror movie. Another benefit is the stand alone and episodic nature of the series - if one story is not to your liking, the next isn't too far off.
It amazes me that these excellent tales are often cast aside in favour of the more child friendly Inu-Yasha and Ranma 1/2. If you're looking for dark, gritty fantasy you could do a lot worse than this, and not a lot better. The only real drawbacks are that there's only four volumes and the stories can get a little repetitive. Definitely a must have little series!