Post traumatic encephalopathy.
...from a fight in 1998 hosted by the Pride organization with considerably different rules that is now closed.
It's also undetermined if the CTE damage wasn't caused by an even longer career of kickboxing, a related sport to UFC but also with different rules and strategies. (and damage risk)
The UFC rules have morphed and changed over the years, and more often than not I've seen a fight stopped early (or my interpretation of early) in order to adhere to the overall rule of protecting the fighter.
I said it was hard to find a career-ending injury. And I was right. You found one in twenty five years of the sport.
I'm not some huge fan of UFC either, but I am aware of the difference that organization has brought to MMA in the way of safety, as their track record in a violent sport speaks volumes.