The Flamewrought Key is required to access Heroic mode for Hellfire Citadel instances (Hellfire Ramparts, Blood Furnace and Shattered Halls).
The Flamewrought Key is sold by Logistics Officer Ulrike for 9g, and requires honored reputation with Honor Hold.
TBC Patch 2.3.0 (patch date::13-Nov-2007): Changed amount of reputation required.
Went from revered to honored.
Cataclysm Patch 4.2.0 (patch date::28-Jun-2011):
All heroic BC instance keys have been discontinued. All heroic BC instances can now be entered at any reputation level.