This is a great thing - this ritual. At the cost of some health and movepoints, it will increase power of next few nightblade combat spells. How much, that depends on power of the night. When you use this spell only a few minutes after sunset, it will be very weak, but in the deepest night it is strong, really strong. The power of this ritual lasts depending on which combat spells you use - more powerful spells will deplete it faster than weaker ones.