Shadowfrost Shard

Shadowfrost Shards are splinters from the Lich King's throne that scattered throughout the citadel when Arthas Menethil struck Ner'zhul's prison with Frostmourne to release his soul.

Source
The fragments are dropped by bosses, or their chests, in Icecrown Citadel on 25-player normal and heroic difficulty only. The drop rate is approximately 25-50% for normal mode bosses and 51-99% for Heroic mode bosses.