تَرْهَقُهَا قَتَرَةٌ
Sentence structure
تَرْهَقُ is the verb, its attached pronoun the object referring to the faces, and قَتَرَةٌ the delayed subject. The whole clause may be a second adjective or a circumstantial clause.
Ayah summary
Darkness and gloom will overwhelm them.
تَرْهَقُهَا — Will cover them
Grammar: third-person feminine singular imperfect verb. Attached هَا is a third-person feminine singular pronoun functioning as the object.
قَتَرَةٌ — darkness
Grammar: nominative noun functioning as the subject.