Boots of Etherealness

The Boots of Etherealness allows the wearer to become ethereal once per day for 5 rounds, making him or her immune to normal weapons, while still being able to cast spell and use weapons.

Baldur's Gate II: Shadows of Amn

 * City of Caverns - Reward for the Riddle of the Imps quest

In-game description
"Why these boots were ever made has long been the subject to speculation among the sages of Candlekeep. Some suggest that a certain noblewoman was gifted them by an ethereal lover so that she could join him for a tryst. Other simply point out that the boots are useful for any mage to become intangible to attacks while devastating an opponent with offensive magic."