﴿ حَتَّىٰٓ إِذَا فَتَحْنَا عَلَيْهِم بَابًۭا ذَا عَذَابٍۢ شَدِيدٍ إِذَا هُمْ فِيهِ مُبْلِسُونَ ﴾

“until We open before them a gate of [truly] severe suffering [in the life to come]: and then, lo! they will be broken in spirit.”

Until hattā is for inceptiveness when We opened on them the gate of a severe chastisement namely the day of Badr slaying them behold! they are aghast thereat despairing of anything good.