It is the subsequent event as splitting and breaking of the sky. This is mentioned by Allah (swt) in the Holy Qur’an:

 ﴾فَإِذَا انشَقَّتِ السَّمَاء فَكَانَتْ وَرْدَةً كَالدِّهَان ﴿

(Then when the heaven is rent asunder, and it becomes rosy or red like red-oil, or red hide) (Ar-Rahman : 37)

Therefore, there will be a universal upheaval with regards to the earth and the sky as the sky is connected to the Earth.

Then what is the situation of mankind at that instance?