Surah Al-Qamar Ayat 16

Allah memperingatkan orang yang membangkang dan mendustakan para rasul serta tidak mengambil iktibar terhadap dahsyatnya siksaan Tuhan dan ancaman-ancamannya yang ditujukan kepada orang-orang yang tidak mengindahkan seruan para rasul

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’; the intention is to prompt those who are being addressed to affirm the fact that God’s chastisement of those who denied Noah was fully deserved.