Greater Invocation; 5th
The dark powers cloak you and shield you from harm while draining vitality from nearby foes. This invocation grants you total concealment in any area that isn’t brightly lit (it will not work in daylight or in the radius of a spell with the light descriptor). In addition, any living creature adjacent to you with this ability active must make a Fortitude save at the beginning of its turn or take a –4 penalty to Strength for 5 rounds. Once a creature is affected by enervating shadow, it cannot be affected again by your enervating shadow for 24 hours. The duration of this ability is 5 rounds, and it can be countered or dispelled by any light spell or effect of equal or higher level.
Source: Complete Arcane