The Umisen-Yamasen (海千山千) are a type of Mushi which are sub-categorized by where they live. Both look like small, snake-like creatures but one lives in forested areas while the other lives in the ocean. After one thousand years both of the Mushi come together to combine to make a single Mushi. The ones living in the sea can create a mist that anyone can see out of so long as they want to get back to shore.
Their weakness is unknown.
This Mushi's name comes from the Japanese expression 海千山千 (umisenyamasen), literally meaning "ocean-thousand, mountain-thousand". This expression is used to refer to a person who has had enough life experience to get out of any situation using their wit. It refers to an old saying that a snake must live one thousand years in the ocean and another thousand years in the mountains before becoming a dragon.