S2·E19 "Empty of Naught"

Hollow quickly proves to be Geo's toughest opponent yet. His attacks are blinding fast and no matter how many times Geo lands a blow, Hollow shows no signs of damage. Aaron and the Satella Police soon arrive on scene, and Aaron and Tom locate the source of Hollow's unstoppable power, a device located within Vega's car. After Copper destroys it with his signal jammer, Geo finally manages to harm Hollow, but Vega uses the two OOParts in her possession to heal him. With the three artifacts in such close proximity, a portal to Mu appears in the sky.