Surah 56. Al-Waqia, Ayah 1

I.e., the Last Hour and Resurrection.

The Event Inevitable is the Hour of Judgment. People may doubt now whether it will come. But when it comes, as it will come, suddenly upon them, it will come with such tremendous reality that it will be burnt deep into the experience of every soul. No one can then be deceived or entertain false notions about it.