The literary technique used for what the question asks is peripeteia.
It´s a turning point, a change that made the action goes to its opposite, according to Aristotle it is one of the most effective techniques in drama and tragedy.
When the peripeteia happens is the moment when the plot starts to move steadily to its conclusion. In the tragedy is the moment where the hero´s fortune change from good to bad.
I hope this answer helps you.
