الظُّلمُ ظُلُماتٌ يَوْمَ الْقِيامَةِ
The oppressor will be in darkness on the day of judgement, in the fields of hashr (gathering), the oppressor will be surrounded by darkness, he will be completely devoid and exempt of the light (noor) that will be given to the mu’mins (true believers)
As Allah mentions in the Quran:
نُوْرُهُمْ يَسْعى بَيْنَ أَيْدِيْعِمْ وَ بِاَيْمَانِهِم
On the day of judgement they will have noor running in front of them and on their sides.
With this light they will reach their destination, meaning jannah (paradise).
Some ulema have mentioned that ظُلُمَاتmeans the hardships and trials of the day of judgement, oppressors will be drowned within such hardships on the day of judgement.
Oppression is such a vile thing that it destroys the life of the oppressed,
Islam wishes that people have affection and brotherhood between them, oppression ends all this.
In a hadith Qudsi Allah mentions:
يَعِبَادِيْ انِّيْ حَرَّمْتُ الظُّلْمَ عَلى نَفْسِيْ وَ جَعَلْتُهَ بَيْنُكُمْ مُحَرَّمًا فَلا تُظَالِمَ
“Oh, my servants! I have made oppression haram (forbidden) upon myself, and also between you. So do not oppress each other.