Megaera, the grudging one, is one of the three Erinyes as Vergil names them (Aeneid 6.240, 12.845), the earliest source for the triad's names. The Erinyes themselves are older and usually nameless: Hesiod has them born from Ouranos' blood on Gaia (Theogony 183-187), and the same poem also makes them daughters of Nyx. Their work is the enforcement of oaths and of the claims of blood, which the Iliad invokes at 19.259-260, and Aeschylus stages them as the prosecutors of Orestes in the Eumenides of 458 BCE.
Modern Practice
The power of the kept account: the resentment that is accurate, and the envy that has stopped being useful. Called on to see a grudge plainly rather than to act it out - the Erinyes' vengeance, once started, does not consult the convenience of the person who started it.