How do you spell Darkening

Available Definitions:
1)  n. - Twilight; gloaming.
2)  a. - To render dim; to deprive of vision.
3)  a. - To cloud, obscure, or perplex; to render less clear or intelligible.
4)  a. - To cast a gloom upon.
5)  a. - To make foul; to sully; to tarnish.
6)  v. i. - To grow or darker.