Skill Effect Level

It lowers the level that your skills are considered while you are attacking or healing. So if you had Dread -12 your skills would be considered 10 levels lower. This causes your skills to become less potent (as if you were 10 levels lower than you are in the previous example). This will also give your skills a higher chance to miss because the skills are considered lower level. This is what makes the debuff the most dangerous IMO.