Amulet of Spell Shield

Blocks a negative spell that an enemy casts on the Hero once every 40 seconds.

This hemispherical disk is made of an unknown material that is not quite metal, but seems just as sturdy and solid. The translucent surface appears to swirl and dance with vibrant colors hiding deep within its body.

Source
Malfurion, Tyrande and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.