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.
the time at which a situation starts to change in an important way
Source: Cambridge Dicionary
Epiphany is sometimes used in this sense:
a moment when you suddenly feel that you understand, or suddenly become conscious of, something that is very important to you
Note that this is a sort of metaphorical extension of the religious meaning:
a Christian holy day in January (traditionally 6 January in Western Christianity) that celebrates the revelation of the baby Jesus to the world