The Core was a biomechanical, artificial hive mind and the true master over King Andrias. It was created long ago from an amalgamation of the smartest minds from across Amphibia's history. The Core lived under Newtopia Castle in a secret basement in wait until the day when the Calamity Box was returned to Amphibia, while acting as the hidden power behind the Newtopia crown during this time.

After years in dormancy, the Calamity Box finally presented itself when the prophesised warriors arrived in Amphibia, who were destined to end the Core. Upon this discovery, the Core sought to reclaim the Box and undo the prophecy by having its conscience transported to Marcy's body through a helmet, leaving the deactivated mechanical body behind and creating a new identity, Darcy. With one of the warriors as its host, the Core regained control of Newtopia, and launched an invasion campaign on Earth as it planned to a millennium ago.

In "All In," Marcy was freed from the Core's influence after Sasha used her sword to sever the cord connected to Darcy's helmet. However, the Core managed to survive inside Darcy's helmet and escaped. In "The Hardest Thing", after making its escape, the Core ended up initiating its final gambit by connecting to Amphibia's moon and setting a collision course with said planet. Though the Calamity Trio used their powers to destroy the Core's drones, it was not enough to stop it, with Anne ending up absorbing Sasha and Marcy's gems to shoot a powerful blast that destroyed the moon and the Core, ending its reign of terror for good.