In the story, it is Hades who holds Percy's mother (Sally Jackson) hostage.

Earlier in the story, Zeus's master bolt is missing, and everyone suspects that Hades has it. However, Zeus believes that Percy stole the bolt (on behalf of his father, Poseidon). Athena, the goddess of war, is supporting Zeus in the conflict. Meanwhile, Apollo, Aphrodite, and Ares are firmly in Poseidon's camp.

It is Ares who reveals that Hades is keeping Percy's mother in the Underworld. For his part, Hades believes that Percy stole both Zeus's master bolt and his own Helm of Darkness. The god of the Underworld thinks that Poseidon has used Percy to steal his most powerful weapon. So, Hades kidnapped Percy's mother in order to lure Percy to the Underworld.

Unbeknownst to Percy, the master bolt is in his backpack, planted there by Ares himself. For his part, Hades thinks that Percy will use Zeus's master bolt as a bargaining chip.

Knowing that he does not have the helm of darkness, Percy has to leave the Underworld. Back on earth, he confronts Ares and engages him in battle. Eventually, Ares gives up the helm of darkness, and Percy brings it back to the Underworld. Later, Percy learns that his mother is alive and well.

Who attacked Percy and his mom in Percy Jackson and the Olympians: The Lightning Thief?

The Minotaur, Pasiphae's son, is the creature which attacks Percy and his mother.

The adventure begins when Percy's best friend, Grover Underwood, shows up  at Percy's home one night, claiming that something is chasing him. Percy can't believe his eyes when he sees his friend: Grover is a goat from the waist down, and he has cloven hoofs for feet. Percy soon discovers that his mother knows Grover was keeping an eye out for him for Percy's own safety.

Mysteriously, Grover tells Percy that it is better for him not to know too much about monsters. He also tells Percy that the Mist is often put over human eyes to mask any knowledge of the creatures. When Percy demands to know what is chasing them, Grover states that it is 'the Lord of the Dead and a few of his blood-thirstiest minions.'

Percy's mother, Sally, frantically drives Grover and Percy to Camp Half-Blood. She tells Percy that he will be safe there. However, the car is struck by lightning not too far from the camp, and it ends up in a ditch. As the Minotaur comes up to the car, Sally desperately urges him to climb out through the passenger's side and to make for the big farmhouse down in the valley after he crosses the property line at the nearest hill.

When Sally tells him to go without her, he refuses. In the end, Percy and his mother help the badly-wounded Grover towards the property line. Sally advises Percy to turn directly sideways as soon as the Minotaur comes right up to him; she tells him that this is the only way to avoid capture, as the Minotaur cannot change course easily once he starts charging ahead.

Meanwhile, Sally sets Grover down in the grass and starts retreating down the hill in order to lead the Minotaur away from both Percy and Grover. The Minotaur starts charging after Sally, and she tries the sideways maneuver to escape being captured. However, the Monster has since learned his lesson and refuses to be outwitted this time; it reaches out its arm and grabs Sally by the neck. As the Minotaur prepares to strangle Sally, she disappears, first melting into a 'shimmering golden form' and then fully vanishing after a blinding flash. The Monster then turns its attention to Grover.

Enraged by the chain of events, Percy turns to engage in a battle to the death with the Minotaur.