More like the energies from defeating her were absorbed by the Watcher in the Dark. It's a bit difficult to explain.

[His gaze has turned inward, even as he's looking at the mural.]

Nyx is a force of nature, Brig. A natural part of the machine of the universe. All things atrophy, all things eventually end. She is the one who brings that end, and carves out the means for the door to the next world to open.