Splinter of Atiesh

A Splinter of Atiesh is a tiny piece of, which was last wielded by the Last Guardian, Medivh. Once all forty splinters are found, they may be assembled into the Frame of Atiesh.

Source
This item is a common drop from all bosses in Naxxramas except Sapphiron and Kel'Thuzad.

Background
After Medivh's defeat, the mages of Kirin Tor searched the tower of Karazhan for any magical items. Finding, they brought it back to Dalaran for study. However, every mage that came into contact with the staff tragically died. The Kirin Tor decided the best course of action would be to place the staff behind impenetrable barriers of magic. Unfortunately when the demon Archimonde destroyed Dalaran, the barriers broke and the staff was shattered into many pieces. Kel'Thuzad now seeks to rejoin the staff to gain enormous power.