The Nightar themselves aren't even sure how it is they can pull energy directly from shadows, and effectively assume it must therefore be Divine in nature. Conventional belief is that 'She' (whoever this might be is unknown) is responsible for their 'Gift of Shadow' and bestows upon their people the very ability to speak with the darkness of the void. This strange ability lets them directly affect themselves
or others through control of (somehow, against all reason) shadows. I also believe it is likewise the very reason they are cursed with severe allergy to light. Nature (or science) doing a balancing act is far from rare throughout the galaxy. Whatever the case, it is truly amazing. I've seen one of their warriors use this ability before, and it was breathtaking.
Once he had completed his...meditation, I suppose you could call it...once that was completed he was able to forge a weapon directly from shadows and somehow solidify them into a weapon.