For those who have not completed Xenoblade Chronicles, this page or section may contain spoilers regarding the plot. Discretion is advised.
The Mechonis Core (機神界中枢, Kishin-kai Chūsū) is an area in Xenoblade Chronicles. It is the final area within Mechonis, and is the place from which Mechonis' movements can be manually controlled. According to Vanea it is located directly above the Apocrypha Generator, which suggests it is in the upper chest or neck of the Mechonis. It is also the smallest area in the entire game.
The Mechonis Core is where the final fight with Egil takes place, as well as his subsequent truce with Shulk and explanation on what happened to Mechonis. It is also where Dickson is revealed to be a traitor, and Zanza emerges from Shulk's body and kills Meyneth. After this series of events Egil uses the last of his life to make Mechonis attack Bionis, allowing the party to escape, before the whole Mechonis is destroyed.
After this point in the story all Mechonis areas except the Fallen Arm will become inaccessible. Also, Alcamoth will become filled with Telethia and inaccessible via skip travel; however, it is still accessible through the Centre Gate. Many timed quests (mostly from Alcamoth) will expire, although new quests will become available. Some new Unique Monsters will appear in some places, along with all the Superbosses except the Final Marcus, which is already present.