The relation between 'crucifixion' and 'cruci fiction' is that 'cruci fiction' is probably a derivative concept. Since crucifixion has such a strong cultural and religious impact, it can be a fertile ground for creating fictional works. These fictional works, which we could call 'cruci fiction', can range from re - interpretations of the historical crucifixion events to completely new stories set in fictional universes that still use the idea of crucifixion as a starting point. It's like 'cruci fiction' is the fictional exploration of the very real and impactful concept of crucifixion.
Crucifixion is the real - world, historical and religious concept. 'Cruci fiction' is related to it in the sense that it might be stories or ideas that are inspired by the real event of crucifixion. For example, if there is a fictional story that has a character who experiences something similar to crucifixion in a fantasy world, that would be 'cruci fiction' which is related to the original concept of crucifixion.