Shinka needed to return and they had already bid each other farewell, but Adam stayed staring at her, curious to find out how exactly she was going to return to the underworld.
She could see that and knew that it was a secret of the underground, but after forming a Soul Contract and speaking with him for such a long time, she trusted him and had no reason to hide it.
There was something that Soul Creatures and especially the Death Corps were skilled at, and that was creating portals and wielding space itself.
Although Shinka wasn't particularly skilled at it and couldn't be described as a skilled, capable or notorious individual within the Death Corps, all members of the Death Corps were capable of forming a single portal.
To do so, they used a Space Crystal that was given to them by the Death Corps and after the secret incantation was recited, along with large amounts of Soul Power being poured into it, a portal back to the underworld would be formed.