Iconic Rumiko. Some of the ideas (and characters? Sengoku zombie Rin, anyone?) have been recycled in different forms in later works. Her view of mermaids is... interesting. But on a deeper level it deals with the problems of immortality in a world full of mortals and some of the psychological stuff, mingled with a good deal of horror (what happens when a serial killer becomes immortal? when immortals love mortals? when the only way to become immortal is by feeding on someone else?) The Japanese version, at least, is now available on Kindle, which is much better than having to buy hardcopy; sadly, as far as I know, the English version is out of print. JP Kindle link: https://www.amazon.co.jp/s?k=人魚の森&rh=n%3A2250738051%2Cn%3A2293143051&dc&__mk_ja_JP=%25u30AB%25u30BF%25u30AB%25u30CA&qid=1571983131&rnid=2321267051&ref=sr_nr_n_4
tl;dr An older, shorter, more psychological version of Inuyasha.
ED: An officlal English re-release! https://www.viz.com/read/manga/mermaid-saga-collector-s-edition-volume-1/product/6474