Well, getting back on topic from the discussion about Hydaelyn...
Implicitly, the power of darkness is harnessed through physical and/or emotional duress, conscious or otherwise. "Fray" is a construct of your post-traumatic stress from 2.x, and as a defender with no shield we're taking a lot of punishment. Blood Weapon drains MP with every hit. Then there's Blood Price, where we literally feed off of our pain, and Syphon Strike, Souleater, Dark Arts Abyssal Drain, and normal Carve and Spit, where we vampirically drain our enemy's life energy.
What does this have to do with the power of darkness?
Demons and voidsent feast on aether, so you could say we're tempting the darkness with our own aether and harnessing it through the pain and suffering that goes along with it, then replenishing our aether by draining it out of our enemies.

Originally Posted by
shinros
Honestly I think our power is void based I think abyss is just another word for it just like chakra is aether. I mean you can't go around saying that you use the power from a place which spits out demon like creatures no one is going to want your help then.
That's entirely the point of being a dark knight. People will shun and hate you for what you are, for what you do. Dark knights are the people who have seen the truth - that the world is a strange and cruel place, and that the only real justice is that of the grave. Instead of sugarcoating what we do with valor, honor, duty, religion, or custom, we have no illusions about such lofty ideals - we kill the corrupt people law can't touch because nobody else will, and if we don't do so they'll just get away with killing and hurting more people.
We know what we are - criminals, murderers. And we know what that means, what it costs, and what it's worth... and choose to fight anyway, because no one else will.
That's the ideal, at least. Sidurgu himself uses that ideal as an excuse to indulge in vengeance, but he seems to be moving toward that ideal after the 58 quest. Let's just let this little exchange between Yuri and Flynn from Tales of Vesperia sum up the Dark Knight ideal; Yuri is, obviously, the Dark Knight.
Yuri: The laws have always been the tools of whoever happens to hold all the chips.
Flynn: Just saying that doesn't make it right for individuals to judge right from wrong, or for you to operate out of your own private court of law! If the laws are at fault, then fixing those laws comes first.
Yuri: But you can't deny that lives were saved because those bastards were put down. You'd rather tell those people, 'Sorry you have to die today. I promise we'll change things real soon'?
Flynn: That's not what I'm saying at all!
Yuri: They're out there, you know. People so bad they'll just go on hurting others. What can the average person do but be victimized by people like that?
Flynn: Even so, Yuri, what you're doing isn't right. Do you intend to rain your justice down on all the villains of the world? That's the behavior of a common criminal.
Yuri: I recognize it for what it is, and I've made my choices. Murder is a crime.
Flynn: ...And still, you intend to dirty your hands?
Yuri: Intend to? I already have.
... and that, ladies and gentlemen, is what it means to be a Dark Knight.