Assassins, Caste of (caste), those who kill for pay; caste color is black. Also known as the "black caste".
You will see the black banner of the Caste of Assassins
Book 1, Tarnsman of Gor: Page 175
...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