This item is found only in the console version(s) of Terraria.
The Shadow Mummy Banner is a banner dropped by Shadow Mummies. It resembles a Shadow Mummies' face. It serves no purpose other than decoration in the background, similar to paintings, however, it must be hung from the ceiling instead. Since this is a legacy item, from the Terraria: Console Edition, and the mob is no longer in the game, this banner does not give any status effects.