Mythology

I don't know where I have read soemthing about the myth and genesis of
human kind where one of the brother killed the another and he became
banned by the counceil of gods, he lost his imortality and is the
forefather of human kind. His name should have been changed from "Manus"
to something like "Serymanus" or kind of this. Does someone remember
about a such myth and the correct name of the guy who lost its
immortality?
I tried to find on web something but a query with "serimanos" did not
helped.

Alex