الظُّلْمُ ظُلُمَاتٌ
Meaning: Injustice is darknesses.
الظُّلْمُ is the mubtadaʾ and ظُلُمَاتٌ is the khabar. One word names the action; the second describes its consequence through a striking expression.
يَوْمَ الْقِيَامَةِ means “on the Day of Resurrection.” Here يَوْمَ is an expression of time (ẓarf zamān), and الْقِيَامَةِ is linked to it by iḍāfah. Thus we read the two words together.
ظُلُمَاتٌ is plural; in this context it should not be confused with ظُلْمٌ (injustice).