Bindings of the Windseeker

Thunderaan, Prince of Air, son of Al'Akir the Windlord was attacked by Ragnaros the Firelord, in an attempt to heighten the already impressive power the fire elemental held. Ragnaros succeeded, though Thunderaan's power could not be completely taken into his form. What remained of Thunderaan was placed in a talisman of elemental binding, which was broken into two pieces, then given to the Firelord's two lieutenants, Baron Geddon (left half), and Garr (right half). These two halves are known as the Bindings of the Windseeker.

The bindings are required for Thunderaan the Windseeker (quest), leading up to the forging of Thunderfury, Blessed Blade of the Windseeker.

Source
The left binding drops from Baron Geddon, and the right from Garr in Molten Core.

The estimated drop rate is 3%.