Roc's Core Crystal

Roc's Core Crystal is a unique Core Crystal in Xenoblade Chronicles 2. It is obtained at the beginning of Chapter 4 following Vandham's death. The Core Crystal cannot be used until certain story progress has been made. Only Rex can resonate with Roc's Core Crystal. Upon doing so he will re-awaken the Legendary Blade Roc. When Roc is re-awakened, he will retain all progress on his Affinity Chart and current Core Chip.

Upon entering New Game Plus, Roc will return to Vandham. Once the player reaches Chapter 4 again, they will re-obtain Roc's Core Crystal.

Story
During the events of Chapter 3, Roc returns to his core after Vandham sacrifices himself to save Rex and the rest of the party. Rex holds onto Roc's Core Crystal. Later, when the party is preparing to depart from Argentum for Mor Ardain, Roc's Core Crystal begins to glow again. However, before the party leaves a Gormotti kid, Rhys, pickpockets Rex, stealing Roc's Core Crystal. The party peruses the thief, ultimately tracking them down to Umon's Shipyard in Gormott. There the party finds a group of kids who explain that they stole the Core Crystal in order to get their revenge against a group of bandits who attacked their village. Rex promises to take care of the bandits for them. While traveling to the Brigands' Hideout, Rex resonates with Roc in order to clear and Ether Miasma.

Trivia

 * Roc's Core Crystal is one of only two Unique Core Crystals which cannot be opened until certain progress has been made in the Main Story, the other being the Beastly Core Crystal.