Surah Al-Kahf 18:2

قَيِّمًا لِّيُنذِرَ بَأْسًا شَدِيدًا مِّن لَّدُنْهُ وَيُبَشِّرَ ٱلْمُؤْمِنِينَ ٱلَّذِينَ يَعْمَلُونَ ٱلصَّـٰلِحَـٰتِ أَنَّ لَهُمْ أَجْرًا حَسَنًا

[He has made it] straight, to warn of severe punishment from Him and to give good tidings to the believers who do righteous deeds that they will have a good reward [i.e., Paradise].

Translation: Saheeh International

Tafsir

a Book upright qayyiman is a second circumstantial qualifier for added emphasis to warn of to make by this Book the disbelievers fear severe chastisement from Him from God and to bring to the believers who perform righteous deeds the good tidings that theirs will be a fair reward

Excerpt from Tafsir al-Jalalayn, commentary on 18:2, via Quran.com. The reader has the full commentary and 9 more tafsirs.