I like the almost impervious darkness
It's like the grand villain of the story saying:
- "You are doomed, for I am three-quarter immortal!"
- "Oh, woe is us, what are we going to do? The boss can only quasi-die!"
*Cough* Well, getting back on track, there are fortresses which have been called impregnable, and have fallen after a few weeks of intelligent warfare.
However, calling it impenetrable seems kind of cheesy in this context, since the weakness is so obvious.
I would have gone down the road of cancelling darkness, or even the "void". After all, light can penetrate the void.
This is just thinking along the line of brianpatrick.
But I do feel like it's defeating the original purpose of calling it darkness, so I'll leave it at that until I find a better idea.