"And We will leave some of them on that day to surge over others" means: on the day they emerge from the dam. "And the Horn will be blown, and We will gather them all together." The Sur (Horn) is a horn into which the blower of the Horn blows.
﴿وَتَرَكْنَا بَعْضَهُمْ يَوْمَئِذٍ يَمُوجُ فِي بعض﴾ يَعْنِي: يَوْمَ يَخْرُجُونَ مِنَ السَّدِّ ﴿وَنُفِخَ فِي الصُّورِ فَجَمَعْنَاهُمْ جَمْعًا﴾ وَالصُّورِ: قَرْنٌ يَنْفُخُ فِيهِ صَاحِبُ الصُّور.
