A Morgul blade is a terrifying weapon used by the Nazgûl and their most powerful servants. These blades are cursed with dark magic and can cause wounds that do not easily heal.
Fell Beast Calling
One of the darkest and most powerful uses of a Morgul blade is to call a fell beast. By using the blade, a servant of Sauron may summon one of these winged terrors to serve as a mount.
Note that highly skilled Rangers who have taken the Dark Oath can call a fell beast even without possessing a Morgul blade.