Nether Vortex

Nether Vortex is a crafting material used in high-level Blacksmithing, Leatherworking and Tailoring recipes to create epic items at the 375 skill level of each profession. As of patch 2.4.0, they are used in Alchemy as well.

Source
Nether Vortices drops from all trash mobs in Serpentshrine Cavern and the Eye raid instances. They can also be bought from G'eras for.

Note
These are not Arcane Vortices, which are clouds that wander Netherstorm waiting for someone with a to convert them into Motes of Mana.