Dragonslayer's Signet

Dragonslayer's Signet is an epic ring for caster classes. It gives fire resistance in addition to stat bonuses. It also increases a character's spell critical hit chance.

Source
This item was obtained from the following quests:
 * given by King Varian Wrynn in Stormwind Keep in Stormwind.
 * given by Thrall in the Valley of Wisdom in Orgrimmar.