At the shrine where Amon met the seven clans, Lin Li rushed into the shrine's hall. Right before the door behind the altar was Yuka's Prison Box. The box was open, and above it was the portal leading to Yuka's prison dimension.
Lin Li grabbed the box. While it was still active, she carried the box and ran out of there.
While Amon planned to meet the seven clans who expressed their desire to cooperate, he didn't neglect the possibility of one or more of them having second thoughts. He placed Lin Li outside to watch in case unwanted people approached. It turned out they did.
He had also planned on what to do if this worst-case scenario happened. When he covered the exit with the black cloth, he secretly placed Yuka's box behind the cloth and activated it. The activated portal would release energy that could be sensed by cultivators above the Human Realm, but he had prepared for that.
