Black dagger, (noun); a mark an assassin put on his forehead when he is in service of someone to kill another.
Guardsmen did not detain him, for he wore on his forehead the mark of the black dagger.
Book 5, Assassin of Gor: Page 6
...for he wore on his forehead, small and fine, the sign of the dagger. When he of the Caste of Assassins has been paid in gold and has received his charge, he affixes on his forehead that sign, that he may enter whatever city he pleases, that none may interfere with his work.
Book 5, Assassins of Gor: Page 7