oscurecer
darken
oscuro
dark
+
-ecer
to become (verb ending)
Oscurecer means to darken and can be thought of as oscuro0 (dark0) + -ecer1 (to become1), literally meaning 'to become dark'.
verb
- darkenTo darken or make dark; to become dark.Las nubes comenzaron a oscurecer el cielo. // The clouds began to darken the sky.