Neltharion's Tear

Neltharion's Tear, is a tear of the black wyrm Neltharion, also called Deathwing. This item was probably passed down to his son, Nefarian from whom it drops.

Source
This item drops from Nefarian, final boss of Blackwing Lair.

Lore
Neltharion was the Earth Warder, Aspect of the Black Earth Dragonflight. He was driven mad and became Deathwing, in the process wiping out most of the dragons during the War of the Ancients. During this event, he shed one last tear before going insane.