As the topic describes: drawing the Shadow of Demise card through the infused Door of Shadows makes a permanent copy of the card instead of making it disappear. An unused copy just stays in the hand once the turn ends.
It’s not game-breaking of anything, just a bit of extra value, but definitely doesn’t work as intended. May be due to the special status of the Shadow of Demise, as both the original and the copy have the same visual aura.