Getting the Key
To enter The Shattered Halls, you must first do a chain of quests to get a working key, much like attunement from earlier times.
The key can be obtained by a quest chain starting from the [Primed Key Mold], which drops from Smith Gorlunk in Shadowmoon Valley by The Black Temple.
Alliance
[68] Entry Into the Citadel (Alliance)
Bring the [Primed Key Mold] to Force Commander Danath Trollbane at Honor Hold.
[68] Grand Master Dumphry
Show the Primed Key Mold to Dumphry <Grand Master Blacksmith> right outside (northwest) of Honor Hold.
[68] Dumphry's Request
Dumphry wants you to bring him:
4 Fel Iron Bar
2 Arcane Dust
4 Mote of Fire
[68] Hotter Than Hell (Alliance)
Kill a Fel Reaver and use its fires to forge the key.
Turn [Unfired Key Mold] into [Charrred Key Mold] with dead Fel Reaver.
got this from
www.wowwiki.com/Shattered_Halls_Key