darkening

1. To make dark or darker by reducing light.
2. To become dark or darker (having less light).
3. To get dark (referring to the sky, either in the evening or as a result of cloud).
4. To make dark or darker in colour.
5. To become dark or darker in colour.
6. To render gloomy, darker in mood.
7. To become gloomy, darker in mood.
8. To blind, impair the eyesight.
9. To be blinded, lose one’s eyesight.
10. To cloud, obscure, or perplex; to render less clear or intelligible.
11. To make foul; to sully; to tarnish.

noun

The act of something becoming darker.