In Percy Jackson and the Olympians episode 2, "I Become Lord of the Bathroom," Luke shares with Percy that he and a forbidden child named Thalia took Annabeth in when they found her in an ally. He speaks of her in the past tense and says that she didn't make it so the assumption is that she died, however, it's revealed in episode 3, "We Visit the Garden Gnome Emporium," that Thalia didn't meet an untimely demise but rather was turned into a tree.
Grover tells Percy this after he notes that Annabeth is really going to miss the tree she seems to be saying goodbye to before they head out on their quest. It turns out that's Thalia. In a clash against the furies at the boundary between the human world and Camp Half-Blood, Thalia had turned back to face them to give Luke and Annabeth more time to get to safety.
Before her life was taken, her father Zeus interceded and transformed her into a tree. Percy found it hard to believe that the all-powerful sky god would choose to turn his daughter into a tree instead of doing literally anything else but Annabeth referred to it as a hero's fate. She also seemed to imply that Zeus was trying to protect his child since, as a forbidden child, she would always be in danger.
Whether that was the god's intention or not, living the rest of her days as a tree does mean that Thalia won't be running for her life anymore. It, however, also means she can't live it either or be with the found family she created with Luke and Annabeth. Her sister may be right that this was a hero's fate but it's also a tragic one.
Stay tuned to Hidden Remote for more Percy Jackson and the Olympians news and coverage. I'll be recapping the first season as it streams on Disney+. Check out the recap of episode 1 below!