For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
The Sealed Tower (Japanese: 封印の塔, Fūin no Tō), formerly known as Ose Tower (English dub: /ˈɒseɪ/), is a location in Xenoblade Chronicles. It is located on the lower level of Valak Mountain, near the Harict Chapel landmark. It is of High Entia origin.
The Sealed Tower was built by the ancient High Entia ancestors to seal away Zanza's soul, the Monado, after his defeat at the hands of Meyneth and Egil. About 14 years prior to the events of the game, a team of Homs explorers made an expedition to Ose Tower, and discovered and released the Monado. It was then that Zanza sucked the life force out of the members of the expedition, including a young Shulk. Requiring a physical vessel, Zanza decided to live inside Shulk, allowing the boy to live on borrowed time.
During the trek to Valak Mountain, Alvis recommends that the party should make refuge inside the ruins. Once they reach the entrance, they see that a wall of snow impedes their progress. Eventually they find the Magma Rock to melt the ice and head inside. There, Alvis mentions the tower's connection to the Monado, and the party realises that this was where Dickson found the explorers and Shulk years ago. Alvis talks with Shulk about his visions before they leave to continue the road to Sword Valley.
After the reawakening of Bionis, a group of High Entia refugees uses the ruins as a shelter.
During a Heart-to-Heart - "In Ose Tower" - between Riki and Dunban, Riki mentions that he feels the presence of ghosts within the room. Dunban admits that there is a strange feeling about this room and tells Riki of expedition and the death of Shulk's parents. Riki then responds that he can feel that presences are wishing for Shulk's well-being. This suggest that the souls of those who died during the expedition may yet still linger within the closed room of Ose Tower.