While it's really up to you in the end, the scenario you put out about Nidhogg and the Calamity is a bit impossible. The two events are separated by an extremely large time gap and someone experiencing the Nidhogg battle could not have also experienced the battle with the Garleans during the event of the fall of Dalamud.
 Now if you wanted to have your character witness some kind of dragon attack you could look up the Battle of Silvertear Skies that happened in Mor Dhona. The wiki says it happened 15 years ago, but now with the time skip it would have happened 20 years ago. It doesn't necessarily give you what you are looking for as it seems you are searching for something more Dragoon lore oriented, but it is the closest thing I can see as far as dragon battles go without breaking the lore.Â
Also it was a battle between dragons and the Garleans and it took place in the sky so he would have likely not really participated in the battle, but may have witnessed it. Well that is...unless he fought for the Garlean Empire.
Edit: If you need some help trying to fit your character somewhere I might be able to help if you want to message me. I am not a lore expert, but I know my way around.
 Now if you wanted to have your character witness some kind of dragon attack you could look up the Battle of Silvertear Skies that happened in Mor Dhona. The wiki says it happened 15 years ago, but now with the time skip it would have happened 20 years ago. It doesn't necessarily give you what you are looking for as it seems you are searching for something more Dragoon lore oriented, but it is the closest thing I can see as far as dragon battles go without breaking the lore.Â
Also it was a battle between dragons and the Garleans and it took place in the sky so he would have likely not really participated in the battle, but may have witnessed it. Well that is...unless he fought for the Garlean Empire.
Edit: If you need some help trying to fit your character somewhere I might be able to help if you want to message me. I am not a lore expert, but I know my way around.
"So sharp end faces the enemy. Okay, got it."
- Domino Quixote
- Domino Quixote
