Lost Souls, also called Lost Ones and Mindless Ones in english.
Lost Souls are people who ate mermaid flesh to become immortal but turn into savage, mindless monsters that kills anything in their path. Its a side effect and the price to eat mermaid flesh. The mermaids' blood is poisonous to do this kind of damage. This can affect to animals too, a dog for example. Only the ones who gain immortality without turning into monsters are the lucky ones from suffering this dreadful fate.
Notable Lost Souls
Towa Kannagi (Hand only)
Sakagami Headman/Isago's Husband.