The assassins that carried out the deeds of the Night of the Black Knives were all women, and rumored to be Numen who had close ties with Marika herself.

Sentry's Torch makes all of the assassins visible, even if you don't meet the stat requirements. It is recommended to get this as soon as possible, as their grab attacks can be can deal massive damage and are near impossible to dodge without it. The Ordina assassins are immune to Frostbite, but only while they're invisible. Lesser Black Knife Assassin Lore