The rise of Darth Bane and his creation of the Rule of Two is also a significant event. This new structure for the Sith order changes the dynamic between the Sith Lords. Instead of large numbers of Sith competing with each other, there are only two - a master and an apprentice - which makes their power more focused and dangerous.
The Yuuzhan Vong invasion in the 'New Jedi Order' series is super important. It changes the galaxy in so many ways, from the political landscape to the Jedi's way of thinking. The Jedi have to adapt to this new and very different enemy that doesn't use the Force in the traditional way. This event tests the limits of the Jedi's power and their relationship with the Force.
The fall of the Sith Empire in some of the early Legends novels is a key event. It sets the stage for the long - standing struggle between the Jedi and the Sith.