I'm trying to think of a word which can describe a first encounter that has made a tremendous impact in my life. Unfortunately, I can't think of one. Is there a word to describe such an event?
The first word that came to me was pivotal:
1 : of, relating to, or constituting a pivot
2 : vitally important : CRITICAL
// She is at a pivotal point in her career.
// the report was missing a pivotal piece of information
Generally speaking, it combines both of the above senses. So not only is something vitally important but it also serves as a turning point.
You can look back on an event in your life that had a significant impact, without which things would obviously have turned out differently for you:
It was a pivotal event.
You could similarly say:
It was a key moment.