The enormous door sported an enormous keyhole. A keyhole that was large enough for the unusually large key inside Jack's inventory.
He took out the Themisphere Legacy Key and inserted it into that large keyhole. It fitted nicely. He then turned the key. Several sounds of clicking were heard. He was surprised a single turn of the key produced so many machinations within the door. Not long after, the sounds of clicking ended and the massive door opened ajar.
"Yes! It is indeed for this door!" Jack uttered triumphantly.
He pushed the door to let it open wider. It was very heavy, even for his strength.
After opening the door wide enough, he entered into a completely dark space. The dark seemed to resist his Dragon Eye's night vision ability, so Jack cast illumination. The light from the wisp revealed a wide corridor behind the door.