You're finding out that triggers and lions don't really mix well. Altho planktivorous triggers generally work, O. niger is kind of 50-50, depending on the trigger. However, I can guarantee it was the 100% trigger's fault it got nailed. The trigger may or may not live as a result of the envenomation, and can sometimes suffer long term damage that may shorten its life.
Way back before anyone really knew any better (as in that they don't mix), my picasso got nailed by my adult volitans...it was COMPLETELY paralyzed for about 15 mins (as in floating on the top of the tank on its side). It died within a month, and I firmly believe it was as a result of the sting.
What happened in this case was the trigger tried to hop in front of the lion to steal a piece of food, and it jumped right onto the lion's dorsal spines...oops.
However, what generally happens is that the triggers harass the lions to death.