The Sumerian Soul Stealing Sigil was an ancient Sumerian sigil that allowed the user to steal a person's soul.
After the release of the Darkness, the Crossroad Demon Duke decided to steal souls in order to create a "nest egg" for himself. To this end, he used Gunner Lawless to kill various victims and carve this sigil into their chest so that Duke could collect their souls.
While the exact procedure for using the sigil to steal a soul is unknown, the victim is shown to be killed first and the sigil then carved into their chest. This allows the user to steal their soul. It is unknown if any further steps are needed such as an incantation and if so, what it is.