Word that describes a dramatic device that disrupts equilibrium

Solution 1:

There is the opposite of deus ex machina: diabolus ex machina

Diabolus ex Machina (Demon from the Machine) is the Evil Counterpart of Deus ex Machina: the introduction of an unexpected new event, character, ability, or object designed to ensure that things suddenly get much worse for the protagonists, much better for the villains, or both.

Though it is not a well-established term like deus ex machina. These kind of events can be interpreted as "plot twist" as well.

A plot twist is a radical change in the expected direction or outcome of the plot of a novel, film, television series, comic, video game, or other work of narrative.