This skill sacrifices some of the user's blood in a ritual of darkness, restoring some of his magical power. In effect, hit points and endurance are traded for mana. Constitution, luck, and skill percentage affect the chance of success in this ritual.
As a note, Dark Ritual CAN reduce a cultist to negative hit points, but the cultist remains conscious and may regenerate (and heal) normally, and the cultist may not use Dark Ritual until they are in positive hit points. Thus, it is very rare for a cultist to actually kill themselves with this skill, but it is possible.