Thinking that back-breaking calamity will befall them.
Expecting the worst.
so they might think the Calamity had been inflicted upon them
Knowing that a great disaster will be made to befall them
(They) think/suppose that a disaster/calamity will be made/done with it
Knowing that a back breaking calamity is waiting for them
Knowing that they will be subjected to a torment that breaks the backs
Knowing that a back-breaking calamity will befall them
Thinking that they will be subjected to such a harsh treatment that will break their backbones
Because they will realise that a back breaking calamity is about to befall them
Thinking that some calamity was about to fall on them