What is a suitable word to describe an event that marks the beginning of something terrible?

Solution 1:

The word harbinger could fit here, meaning "a person or thing that announces or signals the approach of another". It may not fit perfectly, though, since a harbinger is typically observed before the event it signals, and isn't usually the instigating event itself. It sounds like the gate opening is itself causing the problem, so if it's causing doom rather than just indicating it, harbinger may not be the best choice. Perhaps there was another indicator of impending doom prior to the opening that people generally refer to as the first sign of trouble?

As a bonus, harbinger is often (but not always) used in a pejorative sense, indicating the onset of something bad - the top 3 Google autocompletes for "harbinger of" are "death", "doom", and "hell".

Solution 2:

omen

an occurrence or phenomenon believed to portend a future event

The dark clouds were considered a bad omen.

https://www.merriam-webster.com/dictionary/omen