Molten Forge of the Deep

Molten Forge of the Deep was lit for a reason and never permitted to go out, and nobody now alive can tell you what the reason was. The forge does not care. It has been running continuously since, on a shift pattern, with a warden and a supply of ore and a workforce, and the fact that its purpose has been forgotten has never once interrupted production. Varden in one room.
Heat has become the only language in here. The crawlers and miners at the entrance are workers first and enemies second, and they attack you the way a shift attacks an interruption. Further in, the distinction between the workforce and the product collapses: cinder golems and obsidian juggernauts are things the forge made out of what was to hand, and what was to hand was mostly people. The Keeper of the White Flame guards a fire nobody has a use for.
Encounters

Tier 1
Forge-Slag Crawler

Tier 2
Magma-Vein Miner

Tier 3
Cinder Golem

Tier 4
Molten Chain Smith

Tier 5
Furnace Warden Dregor (Mini-Boss)

Tier 6
Obsidian Juggernaut

Tier 7
Lava-Tempered Brute

Tier 8
Anvilborn Destroyer

Tier 9
Keeper of the White Flame

Tier 10
Ignar, Heart of the Forge (Final Boss)
The final boss
Ignar, Heart of the Forge is either the forge's operator or its output, and the question is not answerable from outside. When the forge first caught, the ash came down for eleven days and buried a cathedral whole - that was Ignar waking up. Furnace Warden Dregor manages the floor. Ignar simply is the reason the floor exists, and fights with the sheer thermal indifference of a process rather than a person.
Before you enter
The recommended level is 50. Treat it as a starting point: farm earlier tiers, upgrade gear, and adjust your pet when a boss stops you.
Difficulty inside a dungeon comes from one number that climbs with the tier, and the step between tiers is a large fraction of the gap between dungeons. So a wall at tier 7 here is not a signal to grind this dungeon harder - it usually means the opening tiers of the next dungeon are a better use of the hour, and you come back with the gear they dropped.
