I'm not quite sure what you mean by 'a light against the ratkness'. Maybe it's some kind of special power or force in the fanfic.
I think 'a light against the ratkness' in the Warhammer fanfic might be a unique plot element, like a magical artifact or a rare strategy that offers a chance against a powerful enemy. But it really depends on how the author has crafted the story.
It's hard to say exactly. 'A light against the ratkness' could refer to a heroic character's ability to combat a specific evil or threat within the Warhammer universe. Or perhaps it's a metaphor for hope in a dark situation.