Allows a Level 6 Xaerdun witch to apply a second curse to a victim rather than the usual max of one. The second hex will cost a certain amount of health to apply (this health cost is separate from the Sorcery 2 skill Blood Magic).
This is an extremely powerful skill that offers the only way to combine hexes on a victim. For example, a victim affected by both Vulnerability and Flammability will lose both Magic Resistance and Fire Resistance, causing them to take enormous damage from fire. Delving further into the example, a prepared adventurer with 50% Magic Resistance and 50% Fire Resistance would take only 25% of the damage of a Blast of Fire, but if two hexes reduce that to 10% Magic Resistance and 10% Fire Resistance, that same adventurer will instead take 81% of the Blast of Fire's damage. Similarly, a completely unprepared adventurer reduced to -40% Magic Resistance and -40% Fire Resistance would take nearly double damage.
Blood Runes can also allow a witch to apply curses for twin effects rather than stacking one. For example, Fester can nearly eliminate health regeneration while Black Mantle can nearly eliminate healing. A victim affected by both Fester and Black Mantle has no way to regain health, and therefore faces a very grim attrition over the course of a fight of any decent length. Similarly, Slow (which increases the delay between attacks) and Weaken (which lowers melee damage) can combine to nearly neuter a dangerous melee foe.