In his young adult life, Paris was a Messenian Hoppolite, who defended his city against the Spartans. Losing a battle left him wounded but surviving and he was taken in by the Spartans as a slave to work in some iron mines.
One day a big earthquake hit the region and Paris was caved in deep in the mines. That's when he saw the portal open up. Expecting to be dead and crossing into the underworld, he went through it.