Maddgoth's Helm

From Waterdeep: Dungeon of the Mad Mage.

Description
Wondrous item, very rare (requires attunement by a humanoid).

A creature attuned to the helm and wearing it has immunity to all damage while inside Maddgoth's castle, on its roof, or in its courtyard. If the helm is taken from the castle, it turns to dust and is destroyed.