For it (actor -um- & patient) to become dark.
A substantive with actor and patient role is unexpressed in the sentence.
Hemmelong ya un ami immātam ad Bannāwol.
(It) became dark before we arrived at Banaue.
For someone, some object (actor -um-) to cast darkness (theme) on an area, someone, something (loc ref -an) by blocking the light, with its mass.
helngan
Helngan nan būnut di būlan.
The cloud is casting darkness on the moon.
Adim helngan nan tamūa'.
Do not cast darkness on what I am working on (by standing in the light).
-an: helngan