The black mark is very well know in the pirate world .
It is just a peice of black paper who ever gets it die as the last ray of sun disappear on the horizon.
Some believe a traitor from the crew will kill you .
Other thinks a monster will eat you ...Or a see creature!
It is so rare. Very small but famous people had died to it no one knows how it kills but the one thing they know is that it kills .