Hell's Paradise: Jigokuraku Vol. 3 (manga)

Gabimaru is a ninja on death row with one chance to see his wife again--by finding the elixir of immortality on a supernatural island and delivering it to the shogun. Standing in his way are his fellow convicts and the fearsome beasts that roam the island, devouring or killing anyone they encounter. In addition, Gabimaru must contend with sinister beings who have lived on the island for several lifetimes, tinkering and experimenting on humans and the local fauna to further the goals of their dark sciences. Who are these otherworldly individuals that rule this beautiful and deadly place? And how can Gabimaru possibly stand against them?