Thalia is a demi-goddess who died trying to protect Annabeth, Luke, and Grover from the Furies on Half-Blood Hill. She was the illegal daughter of Zeus, born after the Big Three had pledged not to have children by mortal women anymore.
Five summers ago, Grover was sent to take her from the mortal world and bring her to safety in Camp Half-Blood. Being the daughter of Zeus means that she was a very powerful demi-goddess, so monsters could detect her immortal smell from a mile away. On the way to Camp Half-Blood, Grover and Thalia hooked up with two other demi-god runaways: Luke and Annabeth. The Kindly Ones (the Furies) chased and attacked the four of them on Half-Blood Hill. Thalia told Grover to take Annabeth and Luke to safety, and she battled the Furies alone. The Furies killed her, but Zeus took pity on her spirit right before she died and infused a pine tree on the top of the hill with her spirit. Thalia's tree now holds powerful magic that helps protect Camp Half-Blood from intruders.