Improved Black Sky (3.5e Feat)
From D&D Wiki
 Improved Black Sky [Dark]
With experience you have learned to increase the effectiveness of your black sky class feature.
Prerequisite: Darkness Class Feature, Black Sky Class Feature, Dark Knight Level 15
Benefit: When you maintain the effects to create an area of darkness you now deal 1d4 per two dark knight levels of negative energy damage to all creatures in the area, excluding you.
Normal: When you maintain the black sky ability only an area of darkness is produced and no damage is dealt.