The Restrained Essence of Sapphiron

The Restrained Essence of Sapphiron is a trinket that is designed for a caster class such as mages or warlocks. Some would argue that this item is optimal for a dual use class such as a shadowpriest, but such debates are always contentious. Realistically all caster classes can benefit from this item.

Source
This item drops from Sapphiron in Naxxramas.