The Soul Eater Killing Sigil is a sigil capable of killing a Soul Eater.
While the lore states that a Soul Eater can't be killed, the Men of Letters discovered this sigil which is capable of killing one and placed it in their archives. Unlike the sigil that can trap a Soul Eater, this sigil is powerful enough to force all of the trapped souls from the Soul Eater's nest and set them free. The trapping sigil only frees the souls of the living.
In 2016, Sam Winchester discovered the sigil in the Men of Letters archives while looking for information on Soul Eaters and he and his brother set out to use the sigil to kill the Soul Eater. Entering the nest, Dean painted one on the wall inside the nest before he was possessed by the Soul Eater to stop Sam. Sam was able to subdue his possessed brother and finish the sigil. The Soul Eater was expelled from Dean's body and turned to dust as it died. The sigil freed all of its victims from its nest, restoring the living ones to their bodies and allowing the dead ones to move on.
After killing the Soul Eater that had been released, the Winchesters decided to travel to Tennessee and use the sigil to kill another Soul Eater who had been trapped by Bobby many years before.
The sigil is painted in blood, one inside the house the Soul Eater resides in and one inside its "nest" outside of space and time. The one inside the "nest" "sets the table" while the one in the house kills the Soul Eater. One sigil is painted on one wall while the other is painted on a wall directly opposite the first one. Once the sigil inside the house is completed, the Soul Eater will turn to dust. If they are possessing anyone's body, they are harmlessly expelled.