Surah Al-Qamar 54:16

فَكَيْفَ كَانَ عَذَابِى وَنُذُرِ

And how [severe] were My punishment and warning.

Translation: Saheeh International

Tafsir

How dreadful then were My chastisement and My warnings? this is an interrogative meant as an affirmative; kayfa ‘how’ is the predicate of kāna ‘was’ and it is here being used to inquire about a ‘state’; …

Excerpt from Tafsir al-Jalalayn, commentary on 54:16, via Quran.com. The reader has the full commentary and 9 more tafsirs.