embo.oksIn Greek mythology if you bathe in the River Styx you sort of become invincible except for one weak spot ("Anyone who bathes in the River Styx will be gifted with the strength, power, speed, and near invincibility of Achilles. The only spot where one who has bathed in the River Styx is vulnerable is in their Achilles' Heel, a spot of the bather's choosing which will keep them grounded to the real world and their mortal life after they bathe in the river.")
Before Kronos took over Luke's body Luke bathed in the River Styx so his body could handle the titan. This caused Kronos to be practically invincible so Percy went to the River Styx to have more even ground. Percy knew if he didn't he wouldn't stand a chance against Kronos.
