Hesiod describes them as “wonderful children” and recounts how they earned their status and reputation.
Namely, when Zeus was preparing for the Titanomachy, he tried to assemble as many allies as possible, by promising whoever fights with him a divine office and appropriate rights. At the advice of her father Oceanus, Styx was the first one to join his forces, bringing with herself her four children. Zeus repaid her by declaring that, from then on, all gods will swear their oaths by her sacred waters and that Styx’ four children will live with him forevermore.
To find out more about Kratos and his family, check out Hesiod’s account in his “Theogony.” More prominently, Kratos appears as a character in Aeschylus’ “Prometheus Bound” where, at the very beginning, he – together with his sister Bia – drags the captive Prometheus all the way to a rock in the Scythian land, and afterward compels the hesitant Hephaestus to chain him to it.