﴿ فَكَذَّبُوهُ فَأَخَذَهُمْ عَذَابُ يَوْمِ ٱلظُّلَّةِ ۚ إِنَّهُۥ كَانَ عَذَابَ يَوْمٍ عَظِيمٍ ﴾

“But they gave him the lie. And thereupon suffering overtook them on a day dark with shadows: and, verily, it was the suffering of an awesome day!”

Please check ayah 26:191 for complete tafsir.