Blackrock Depths (aka Blackrock Deeps, BRD) is the deepest part of Blackrock Mountain, ruled by the Dark Iron dwarves who are led by Emperor Dagran Thaurissan. Several minions of the Firelord Ragnaros also make their home in these lava-lined depths. BRD is currently the largest 5-man instance in the game, and is likely to remain so, making it a true wonder of the World of Warcraft. The average time to complete the entire instance is 4–6 hours due to its complexity: a large amount of mobs and bosses, several scripts, and many quests. The enemy level range is 48-56, and the minimum level requirement to enter is level 42.
The Anvilrage Military are not to be underestimated either, particularly the Officers (who have Divine Shield) and the Medics who can heal their troops. Even for a lesser-geared level 70, they can be dangerous in large groups.
BRD is not visited as often these days since many players prefer to go to Outland at level 58 (the rewards for Outland's initial quests render BRD's gear obsolete). Farming for Fiery Weapon enchanting recipe may still be useful, since it is commonly used by low-level players.
In Wrath of the Lich King, BRD attracted more interest due to the instance-clearing achievement; and the introduction of Jeeves, which requires Field Repair Bot 74A (which is learned in BRD).
The instance is also almost entirely soloable (for Rogues or Hunters, at least, and probably most melee classes) by about tier 5 (level 70), although it is not economically efficient to do so. Although there is money to be made here (especially for miners), the sheer size of the instance and the comparatively low level of the mobs mean that there are better instances for farming (notably Scholomance).
For level 80, this instance is easily soloable. Fire mages may still have harder time than Warriors with Bladestorm, for example.
Although it is not especially profitable, BRD (and the rest of the Blackrock Mountain complex) is well worth at least one visit. These instances represented the original game's end-game content, and were made with a high level of quality.
Once the capital city of the Dark Iron dwarves, this volcanic labyrinth now serves as the seat of power for Ragnaros the Firelord. Ragnaros has uncovered the secret to creating life from stone and plans to build an army of unstoppable golems to aid him in conquering the whole of Blackrock Mountain. Obsessed with defeating Nefarian and his draconic minions, Ragnaros will go to any extreme to achieve final victory.
To complete the Detention Block, you go straight forward into a large open cavern, then turn right into the cell blocks, and kill High Interrogator Gerstahn at the end of the hallway. She is the boss who drops the Satchel. It's a rather quick trip.
Upper City - 51-61
To complete the Upper City, you have to kill the Emperor. This journey will take quite some time due to BRD's size.
If you have already killed the High Interrogator Gerstahn, you will not get the Satchel on killing the Emperor; you will instead get it by mail. There have been no reports as to what happens to the bonus XP you are supposed to get.
Tip: If your group consists of mostly lvl 52+ chars and Upper City is yellow, while Lower City is green - go for the Emperor without killing the High Interrogator Gerstahn.
Dungeon Finder
Queuing for randoms in Dungeon Finder can either drop you in the Detention Block (Lower City) or Upper City. If you are unfamiliar with the two areas, you may be able to determine your location based on your fellow party members: if you have someone below level 51 in the group, you are in Detention Block; if you have someone above level 57, you are in the upper City.
The depths are inhabited by Dark Iron Dwarves, their demonic bloodhound pets, their golem protectors, and their fire elemental allies. There is also a contingent of Twilight's Hammer ambassadors inside the city and the detention block area.
The dwarves in the Depths consist primarily of warrior and priest types. Outside the Shadowforge City, the dwarves are all elite. Inside the city, one can find groups of one or two elite dwarves or Twilight cultists surrounded by a cadre of non-elite citizens. Some other humanoid races appear in the city and in the Ring of Law.
The demonic bloodhounds in the Depths are renowned for their ability to sniff out stealthed rogues and druids. They will sometimes be grouped into their own pack, and sometimes included in a group with humanoid allies. All of the dogs are non-elite, but their damage capability in large numbers is not to be underestimated.
The golems and fire elementals in the Depths generally fit the stereotypes of their races. The golems are strong and well-armored melee attackers, though some exceptional specimens have the ability to release area-of-effect fire spells. The fire elementals cause large amounts of fire damage with melee attacks and ranged fire spells.
Shadowforge Peasant: casts Fury of Ragnaros (increases target's spell damage by 25 for 30 mins), Fireball, Shoot, Heal, Strike (Normal damage plus extra damage to the target.), Throw Dynamite (Deals fire damage to all enemies in a selected area.) (This mob has a random combination of the spells above.)
Golem Mechanics
Doomforge Dragoon: casts Shoot, Exploding Shot (deals fire damage to the target and other people around the target), Cleave
Doomforge Craftsman: casts Shoot, Rebuild (heals the target golem for 100 damage per sec for 10 secs), Bomb (AoE fire damage on a target area), Throw Wrench (1.5 sec cast, physical damage ranged attack)
Ragereaver Golem: casts Uppercut (knocks the target into the air), Flurry (increases the caster's attack speed) and Enrage (increases caster's attack power.)
Bloodhound: casts Rend, Dire Growl (reduces Strength and Agility by 47)), Ravenous Claw (normal damage plus extra)
Bloodhound Mastiff: casts Rend,Dire Growl (reduces Strength and Agility by 47), Blood Leech (deals damage to the target and heals the caster for 3 times the damage), Ravenous Claw (normal damage plus extra)
Plugger Spazzring, found in the Grim Guzzler, has a few interesting items to sell. The grim patrons are neutral once you enter the Grim Guzzler, however they can become enemies.
Friendly NPCs
Mistress Nagmara, found in the Grim Guzzler, has a single quest available to both Alliance and Horde.
Kharan Mighthammer is imprisoned in the Detention Block. He is an intermediate quest target/quest giver in the Alliance quest chain which rewards you for killing Emperor Thaurissian.
Marshal Windsor is imprisoned in the Detention Block. He is an NPC/quest giver in the Alliance quest chains for Onyxia attunement.
Commander Gor'shak is also imprisoned in the Detention Block. He is a quest target/giver for the Horde quest chain involving killing Emperor Thaurissian.
BRD is one of the two places where Dark Iron ore can be mined reliably, the other location being Molten Core. Since patch 1.7, dark iron ore nodes have begun to appear rarely in the Burning Steppes and the Searing Gorge. BRD is also the location of the Black Forge and the Black Anvil. The Black Forge is the only place where dark iron ore can be smelted into bars. The Black Anvil is required for most crafting recipes that require dark iron bars. As such, a BRD run is essential to almost any blacksmithing or engineering work which uses that rare metal.
Blacksmithing
Scattered around BRD are Blacksmithing Plans which appear as scrolls that can be clicked on with a gear icon. Only a high-level Blacksmith should click on these plans; if anyone without high-level blacksmithing attempts to use them the plans will be destroyed. Opening the scroll will give you one of the following BoP plans: