Ring of Spell Power

The Ring of Spell Power used to be the most powerful spell damage ring in game but did not have any other stats. As it is not unique players can wear two at the same time, however most warlocks or mages take one, and use their other ring slot on band of sulfuras, or a resistance ring (situationally)

Source
This epic ring that has a small chance to drop from Lucifron, Gehennas, Shazzrah, or Sulfuron Harbinger in the Molten Core.