“Oropher was slain in the first assault upon Mordor, rushing forward at the head of his most doughty warriors before Gil-galad had given the signal for the advance. Thranduil his son survived, but when the war ended and Sauron was slain (as it seemed) he led back home barely a third of the army that had marched to war.”
“But there was in Thranduil’s heart a still deeper shadow. He had seen the horror of Mordor and could not forget it. If ever he looked south its memory dimmed the light of the Sun, and though he knew that it was now broken and deserted and under the vigilance of the Kings of Men, fear spoke in his heart that it was not conquered for ever: it would arise again.”
Unfinished Tales, The 2nd age: Appendix B - The Sindarin Princes Of The Silvan Elves
“In the North also there had been war and evil. The realm of Thranduil was invaded, and there was long battle under the trees and great ruin of fire; […]”
The Lord of the Rings: Appendix B - The Great Years
So basically I had this theory that Thranduil wasn’t showing Thorin EVERYTHING. He only unglamored a piece of his face for a moment, we don’t know how far the scarring went. And considering he was definitely blind in his left eye from that scarring, the headcanons…grew.
I have read a lot of theories on Thranduil’s scar and I have loved every one of them. So I imagine he sometimes looses control after he receives bad news and that triggers his anger and so the magic he uses to conceal his face falters and it’s painful as hell. Yes.