A calamitous event is one that leads to a catastrophe — like the calamitous crashing of your parents' car into the garage door.
Calamitous is an adjective that is generally used to describe events, and these events are disastrous or destructive. A failed election can be a calamitous event for a politician, especially if he loses by a landslide. Ever heard of Calamity Jane? She was known for her wild, calamitous behavior in the Wild West during the 19th century. Steer clear of anyone with a name like Calamity.