Memory 1: Thalia's Pine
"C'mon, Annabeth, run!" Luke grabbed Annabeth's hand and pulled her along behind him. He parried away an attack from one of the Furies circling overhead with his new sword, which had already been dented badly. He stabbed upward, and the now injured Fury shrieked and retreated.
Grover frantically played his pipes, making the grass twirl around the scaly trunks of the snake women. Thalia followed close behind them, thrusting her spear into the roiling mass of monsters chasing after them. She held her shield, Aegis, over her head. The monsters had been giving her a wide girth, but even as Annabeth watched, a brave hellhound dashed forward and snapped at Thalia's side. She cried out in pain and stumbled, knocking Grover into Annabeth.
Annabeth tripped. Luke scooped her up in his arms, yelling, "Thalia, take Annabeth! I'll hold them off!"
Thalia pushed Luke away. "No!" she panted, "You go! I'm... not gonna make it. And Luke..." Thalia met his gaze for a long moment, reaching out to touch Luke's arm. Annabeth felt him shudder softly, and he nodded as if a silent message had been passed between them. Thalia leaned in and pecked them both on the cheek before turning to face the monsters with a harsh cry.
Holding Annabeth tightly in one arm and dragging the now unconscious Grover with the other, Luke jogged up the hill. Annabeth craned her neck to see over Luke's shoulder. She could see the top of Thalia's head, bobbing and weaving as she fought.
But then Thalia's head dropped, and Annabeth heard the triumphant calls of the Furies. She screamed. "Luke! You have to help her!" She pounded on his arm, but Luke kept running. His face was streaked with tears.
Below the trio, the valley was filling up with kids and teens, pointing up the hill. At first, Annabeth thought they were pointing at Luke dragging Grover and carrying her. But as they started to scream and back away, she twisted in Luke's grasp to see the hilltop glow a brilliant, electric blue. A crash, like a thousand thunderclaps, and then...
A tall, voluminous pine tree stood at the crest of the hill. Sand-like ashes piled at its bulging roots. There wasn't a monster left in sight. Luke stopped, and Annabeth wriggled out of his hold. They stared in stunned silence at the pine tree that had once been their friend.
So this is what I think might've happened when Thalia turned into a tree. I will go ahead and apologize for the holes in the story: still editing/writing some of the memories. Apologies if I skip around. Everything will be updated in a few hours (at the least. Could be tomarrow.)