Fragment of Val'anyr

A Fragment of Val'anyr is a tiny piece of Val'anyr, Hammer of Ancient Kings. Once thirty fragments are found, they may be assembled into the Shattered Fragments of Val'anyr.

Source
The fragments are dropped by bosses, or their chests, in Ulduar on 25-man difficulty only. The drop rate varies from 3% to 100% depending upon the boss defeated.