After Percy wounds Ares in chapter 20 of The Lightning Thief something strange happens: time slows down, the sun seems to darken, the temperature drops and the sounds muffles as a sense of despair embraces Percy. It lasts only a second, and everything afterwards comes back to normal, only leaving Ares a little disoriented.
He then goes away, and Percy and Annabeth briefly discuss whether that sensation was produced by the Furies or something else. It is then when Percy understands that what was trapped in the bottom of Tartarus was Kronos.