Not Yours To Conquer

While you are wielding your possessed weapon, whenever you make a saving throw against a mind affecting effect, the inhabitant of your possessed weapon spawns a defensive barrier within your mind which grants you a +3 bonus on that saving throw; starting at 8th level, "a +3 bonus" becomes "a +6 bonus". Additionally, if you pass that saving throw, the inhabitant of your possessed weapon strikes back at the originator of that mind affecting effect and deals 1d6 points of damage to them in retaliation; starting at 8th level, "1d6 points of damage" becomes "2d6 points of damage".
Note: This hex can only be learned by a corrupted soul.