Shadow World

The Shadow World, also referred to as the Sea of Shadow and many similar names, is an afterlife in the Covenant religion not dissimilar to Hell. While the souls of the virtuous are believed to be recycled through the process of reincarnation until the day in which they can ascend to godhood via the Great Journey, the souls of the traitors are trapped in the Shadow World.

Slipstream Space
When Slipspace travel became possible, the Covenant Prophets declared that the Slipstream Space through which their ships passed was in fact the Shadow World. The two concepts would later become indistinguishable, and led to the process of entering Slipspace being termed "penetrating shadow".