Shattered Halls Key

Shattered Halls Key opens the gate to The Shattered Halls, the third instance in Hellfire Citadel in Hellfire Peninsula. The gate is outside the instance portal, so only one player in the party needs the key (alternatively, a rogue with a lockpicking skill of 350 can open the door).

Source
The quest chain for the key begins by killing Smith Gorlunk in Shadowmoon Valley by The Black Temple. He drops. Click the to obtain the quest Entry Into the Citadel.

Alliance

 * Entry Into the Citadel (Alliance)
 * Bring the to Force Commander Danath Trollbane at Honor Hold.
 * Grand Master Dumphry
 * Show the Primed Key Mold to Dumphry right outside (northwest) of Honor Hold.
 * Dumphry's Request
 * Dumphry wants you to bring him:
 * 4 Fel Iron Bar
 * 2 Arcane Dust
 * 4 Mote of Fire
 * Hotter than Hell (Alliance)
 * Kill a Fel Reaver and use its fires to forge the key.
 * Turn into  with dead Fel Reaver.

Horde

 * Entry Into the Citadel (Horde)
 * Bring the Primed Key Mold to Nazgrel at Thrallmar.
 * Grand Master Rohok
 * Show the Primed Key Mold to Rohok in Thrallmar.
 * Rohok's Request
 * Rohok wants you to bring him:
 * 4 Fel Iron Bar
 * 2 Arcane Dust
 * 4 Mote of Fire
 * Hotter than Hell (Horde)
 * Kill a Fel Reaver and use its fires to forge the key.
 * Turn into  with dead Fel Reaver.