Access via The Lyceum or Summoner's Tomb.
Two groups of fire elementals guard the bridge - on the far side, the swirling portal to Molten Core.
Tank one of the elementals and disable the other two with traps, etc. as they crit very hard.
On the north side of the bridge is a narrow catwalk, which winds into a lavafall. At the end of this catwalk, guarded by a fire elemental, is The Black Forge.
This is the only place where Dark Iron Ore can be smelted into Dark Iron Bars.
The Core Fragment needed for the quest Attunement to the Core is on the left side, next to the portal.
You do not need to enter the portal to finish the quest. In fact, if you enter the portal you will appear outside BRD after exiting Molten Core again, right next to the quest-giver.