Invulnerability

Invulnerability is an ability seen in magical creatures to be incapable of being wounded, hurt, or damaged.

Beings with this ability

 * The Triple Goddess (assumed) - Being a "Goddess" of the Merlin universe, it can be easily assumed that she's invincible and eternal. Morgana herself stated that no mortal blade could kill her.
 * Dragons - Dragons cannot be penetrated by anything, even magic, but a strike to the heart can hurt it.
 * Griffin - Griffins cannot be penetrated but by a magic-imbued weapon.
 * Questing Beast - The Questing Beast cannot be penetrated by anything but by a magic-imbued weapon.
 * Gargoyles - Gargoyles cannot be penetrated by man-made weapons but magic can destroy them.