Chronos Stone
The Chronos Stone was a stone that was swallowed by the Titan Chronos (more commonly referred to as Cronus) in place of his son, Zeus. He later regurgitated this stone (along with all his other devoured children), and it became endowed with time-traveling properties. However, while the traveller could change the past, all memory of their presence in the past was removed upon their return to their native time.
The Chronos Stone was stolen in the time of Hercules by the King of Thieves, Autolycus. It gave Hercules a chance to correct a wrong from the past before the demigod destroyed it, so as to prevent it from falling into the wrong hands.
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In the non-canon animated film Hercules and Xena: The Animated Movie - The Battle for Mount Olympus, a completely different Chronos Stone had vastly different powers- it could be used to weaken the gods of Mount Olympus and strengthen the Titans.