The Phylactery of Kel'Thuzad

The Phylactery of Kel'Thuzad was a quest item dropped each time Kel'Thuzad was killed in Naxxramas when it was a level 60 raid in the Eastern Plaguelands (similar to the Head of Onyxia, Heart of Hakkar, etc.). It is revealed that Father Inigo Montoy (an Argent Dawn priest from the original game) took the phylactery to Northrend and used it to resurect Kel'Thuzad so he could once again rule Naxxramas.

Rewards
Prior to Patch 4.0.3a, there was a choice of two rewards from the quest, one trinket for physical damage and one for magic damage.

Trivia
The flavor text added in Patch 4.0.3a seems to be poking fun at the extremely few characters that might still have this item in their possession, without ever turning the quest in.