Tarnhelm

The Tarnhelm is a magic helmet in Richard Wagner's Der Ring des Nibelungen. It was crafted by Mime at the demand of his brother Alberich. It is used as a cloak of invisibility by Alberich in Das Rheingold. It also allows one to change one's form:

Alberich changes to a dragon and then a toad in Das Rheingold, Scene 3.
Fafner changes to a dragon after the end of Das Rheingold and appears thus in Siegfried, Act II.
Siegfried changes to Gunther's form in Götterdämmerung Act I, Scene 3.

Finally, it allows one to travel long distances instantly, as Siegfried does in Götterdämmerung, Act II.
The stage directions in Das Rheingold and Siegfried describe the Tarnhelm as a golden chain-mail helmet which covers the wearer's face.

In popular culture

In The Lord of the Rings, Éowyn adopts the name "Dernhelm" when she masquerades as a man before slaying the Witch-King of Angmar; "Dernhelm" is the Old English equivalent of "Tarnhelm."