At some point, Zeus had an affair with Callisto. The circumstances are unknown, but the affair resulted in a young child named Kratos. Kratos was a strong boy and a mighty warrior with unstoppable rage and ambition. Hera, displeased at Zeus for fathering yet another bastard, demanded he kill the boy, fearing only destruction will come from his existence. Zeus, taking pity on the lonely child, refused. Having brutally slain his own wife and child in blind anger, infused by the God of War, Kratos sought to rid himself of the nightmares by devoting his services to the gods of Olympus.
Around halfway through Kratos' penance, Zeus was one of the many gods that fell victim to an enchanted slumber, orchestrated by Morpheus when the sun god, Helios, was knocked from the sky. Kratos eventually discovered that this was the work of Persephone, who intended to use him to empower the Titan, Atlas, in order to destroy the Pillar of the World and bring an end to all life, as well as her own suffering. Fortunately for Zeus and the other gods, Kratos was able to defeat the Queen of the Underworld and save Helios, forcing Morpheus to retreat. What they did not know was that Kratos had been forced to relinquish any hopes of reuniting with his beloved daughter in the process of saving the world, pushing his disillusionment with the gods ever closer to open hatred and rebellion.
Near the end of Kratos' service to the gods, Ares besieged the city of Athens to gain favor from Zeus over his half-sister, Athena. Zeus had forbidden the gods from waging war on each other, so he allowed Athena to receive help from Kratos, who was told that defeating Ares would complete his penance. During his quest, Kratos met Zeus himself within the damaged city of Athens, where he was given Zeus' Fury, the ability to wield and throw powerful thunderbolts. In addition, Zeus, in the mortal guise of the Grave Digger, created a portal to the Underworld through a grave he claimed to be digging for Kratos, allowing the Spartan to escape the realm of the dead late in his adventure.
When Ares seemingly killed Kratos and took Pandora's Box for himself, he yelled to the skies, cursing his father for constantly favoring Athena and asking if Zeus could finally see what his son was capable of. He declared that he would not hesitate to use the box against Olympus itself, but then Kratos, having fought his way out of the Underworld, used a bolt of Zeus' Fury to reclaim it, at last opening the ancient artifact and harnessing its power to destroy his former master. Zeus and the gods guided Kratos to defeat Ares, as he was the only real threat to Olympus. With Ares dead, the Olympians forgave Kratos for the crime of killing his own family (although, to Kratos' dismay, they were unable to remove the horrible memories from his mind), and made him the new God of War.
It is later revealed that Zeus and the other gods fell prey to the evils locked away in Pandora's Box after Kratos opened it to defeat Ares. Apparently, the King of the Gods didn't realize that if Kratos used the Box, the evils would be released. As a result, Zeus became slowly overwhelmed by his personal plague: fear that the cycle of son killing father would repeat itself. Just as Cronos killed Ouranus, and as Zeus himself defeated Cronos, the king of the gods expected that Kratos, infuriated by the revelation that not even the gods could end his nightmares, would be the son who would try to kill him and take the throne of Olympus for himself.