unthinkable scenario

Translations
немыслимый сценарий
The team prepared for every possible outcome, even the unthinkable scenario.
Команда подготовилась ко всем возможным исходам, даже к немыслимому сценарию.
Definitions
An unthinkable scenario refers to a situation or event that is so extreme, unlikely, or shocking that it is difficult for people to imagine or consider it happening.