Twilight Emblem

From Zelda Wiki.org

The "Twilight Emblem" is the mark of Zant's evil magic. It is the same shape as the Twilight Portals created across Hyrule by Zant, and can be seen on the magical barriers which appear when fighting groups of Shadow Beasts at a portal. It can also be seen when battling Zant's Phantom in the Palace of Twilight, on the red globe of magic which he hurls into the air to open a Twilight Portal.

Also, this symbol is seen emblazoned onto the bodies of all the Twilight Monsters created by Zant's magic. When any Shadow creature is destroyed, it will explode in a shower of black shadow-fragments, leaving behind a glowing Twilight Emblem which quickly evaporates.