Peripeteia

Peripeteia is "a reversal of circumstances, or turning point, within a work of literature," from the Greek; peripety is the English variant. In the Poetics Aristotle defines it as "a change by which the action veers round to its opposite," subject to probability or necessity, and ranks it, together with discovery, as the most powerful element of tragic plot. His principal example is Oedipus Rex, where the messenger's news flips the king's fortune in a single stroke. The reversal often runs from good fortune to bad as an ironic twist, and it works best when it coincides with a recognition scene.

For novelists it is the sharp end of a plot twist: the twist changes what the reader knows, the peripeteia changes what happens to the character. It is the beat a character arc bends around, and in a web serial it is best staged at an arc boundary, where a reversal lands hardest.