Waqtaraba alwaAAdu alhaqqu fa-itha hiya shakhisatun absaru allatheena kafaroo ya waylana qad (surah Anbiya: 97)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(21:97) and the time of the fulfilment of the True Promise will draw near, *93 whereupon the eyes of those who had disbelieved will be struck with amazement: they will say, "Woe to us: we were heedless of this: nay, we were wrong-doers. " *94

Waqtarabal wa`dul haqqu fa-izaa hiya shaakhisatun absaarul lazeena kafaroo yaawailanaa qad kunna fee ghaflatim min haaza bal kunnaa zaalimeen

*93) For the details about Gog and Magog, see E.N.'s 62 and 69 of Surah A1-Kahf (XVIII). They will be let loose in the sense that they will swoop down on the regions of the earth like a beast of prey which is set free from its cage suddenly. "The time of the fulfilment of the true promise" will come on the eve of Resurrection and the appearance of Gog and Magog will be a sign of it. In a Tradition related in Muslim, by Huzaifah bin Asid Ghifari, the Holy Prophet said, "Resurrection will not take place until ten signs appear:
(1) The smoke. (2) Dajjal. (3) Daabat-ul-Ard. (4) rising of the sun from the west. (5) coming down of Jesus, son of Mary. (6) sudden attack of Gog and Magog. (7-9) three land slides-one in the east, the second in the west and the third in Arabia. (10) the flare up of a great fire from Yaman which will drive the people to the Plain of Resurrection. In another Tradition the Holy Prophet stated that after the swoop of Gog and Magog, Resurrection will. be so near that it may take place at any moment just as a pregnant woman might deliver the child at any moment, in the day or night, after her term has expired. But the reference to the appearance of Gog and Magog in the Qur'an and Hadith does not indicate that they will swoop on mankind jointly; it may be that they will clash against each other near the time of Resurrection and their mutual conflict might result in a universal catastrophe and destruction.
*94) "We were heedless of this" contains a kind of excuse in it, as if to say, "The Prophets warned us of the Day of Resurrection but we were wrongdoers and paid no heed to their warning".