Whereon the heaven will be cleft asunder? His Promise is certainly to

﴿ٱلسَّمَآءُ مُنفَطِرُۢ بِهِۦۚ كَانَ وَعۡدُهُۥ مَفۡعُولًا ﴾
[المُزمل: 18]

Whereon the heaven will be cleft asunder? His Promise is certainly to be accomplished

﴿السماء منفطر به كان وعده مفعولا﴾ [المُزمل: 18]

Al Bilal Muhammad Et Al
When the universe will be cleft asunder? His promise must be accomplished

Ali Bakhtiari Nejad
(A day that) the sky is torn apart in it. God's promise shall be done

Ali Quli Qarai
and wherein the sky will be rent apart? His promise is bound to be fulfilled

Ali Unal
The sky will cleft open thereby. His promise (of Resurrection and Judgment) is certainly to be fulfilled

Hamid S Aziz
The heaven shall rend asunder thereby; His promise is ever brought to fulfilment

John Medows Rodwell
The very heaven shall be reft asunder by it: this threat shall be carried into effect

Literal
The sky/space is split/cleaved with (by) it, His promise was/is made/done

Mir Anees Original
(and) on which (day) the sky will be cleft asunder ? His promise is to be fulfilled

Mir Aneesuddin
(and) on which (day) the sky will be cleft asunder ? His promise is to be fulfilled