Border of Reality.
"The demons have been awfully quiet lately." The gods were talking in the Legacy Company's front lounge. They were ready to defend the city at a moment's notice, so none of them lost their concentration, even for a moment.
God Zeus and God Odin were seated around a round table along with other major gods, such as Ares, Apollo, Poseidon, etc.
However, even though they were all concentrated, they didn't notice a person walking right past them. The person looked transparent, as if he was donning an invisible cloak. No one realized anything, like he wasn't existing.
He was Underlord, who was walking inside the borders of reality. It made him impossible because he wasn't inside the known reality, so it was no different from not existing.
He took the stairs since he couldn't interact with any objects, otherwise he would be caught quickly. Thus, he ran up the staircase and, after ten minutes, arrived at the top floor.