The Phylactery of Kel'Thuzad

The Phylactery of Kel'Thuzad was a quest item dropped by Kel'Thuzad was defeated in Naxxramas when it was a level 60 raid in the Eastern Plaguelands. Similar to the Head of Onyxia or Heart of Hakkar, one was guaranteed to drop on every kill. In Wrath of the Lich King, it was revealed that Father Inigo Montoy (an Argent Dawn priest from the original game) took the phylactery to Northrend, giving Kel'Thuzad time to regenerate so he could once again rule Naxxramas.

Trivia

 * Kel'Thuzad's phylactery shares an icon with the Urn of King Terenas (which stored his remains during the journey to the Sunwell). As a lich's soul is typically bound to an object during his transition into undeath, it is likely the same urn. Kel'Thuzad's dialogue in Heroes of the Storm confirms this, though the game is not considered canon.
 * Though it was removed in patch 3.0.2, players who had it could still turn it in. Patch 4.0.3a removed the quest and added the flavor text, "Someone has been holding onto this for a VERY long time," poking fun at the extremely few characters that might still have this item in their possession.