The Dark Coffer, also known as "the safe" is one of the major reasons high-level characters are still raiding Blackrock Depths. Basically, it is a chest with various valuable items including greens and gems.
However, the main reason to visit the safe is the 
[Burning Essence], which is needed for the +20 Fire Resistance Libram Enchantment for your Head/Leg slot as you progress through the Molten Core.
Casters may also want to get the 
[Black Blood of the Tormented], for the +150 Mana enchantment.
The Coffer Run
Preparation
Before you head off to BRD you should consider the following:
- You will need a party equivalent to a balanced 5-man group of 60's to efficiently do the run.
- It is possible to stealth farm the Dark Coffer with Rogues and Druids. The number and composition depends on the gear level of the players. A druid and a rogue in Tier 1 gear should be able duo a number of the Dark Keepers.
- Having the
[Shadowforge Key] is strongly recommended, though not neccessarily needed. As of 1.10, conventional means of lockpicking will do the trick as well.
Getting to the Dark Coffer Room
After entering the Blackrock Depths, turn left and have the gate opened either by a rogue or with the Shadowforge Key. Kill or avoid the fire elemental, opening the next gate to the right. Unlock the door and enter the Shadowforge Lock room. Watch out for roaming fire elementals up the stairs. On top of the stairs run straight forward and enter the top level of the arena. With the arena event uncompleted, all spectators will aggro. However, they can be avoided by hugging the inner ledge while crossing the room. Tauren can also try to step upon the ledge to gain even more distance to the spectators.
Once on the other side, fight to the Coffer Room. Turn left and fight to the large Bridge (watch out for the "ambush event") and enter the Domicile. One can try to cross the room to the set of stairs to the left side in the back of the room without aggoing any mobs on the way, but most suggest taking the groups out regularly. Fight through the Domicile to a round-shaped room on a semi-circular ledge. Lord Incendius is below. The Dark Coffer room is beyond the next entrance along the ledge.
Go inside the room and clear any mobs. In the back is the Dark Coffer at the center of the wall (along with numerous Relic Coffers, openable for minor loot using Relic Coffer Keys). Be careful though, since the 4 golems will come to life and attack once all Relic Coffers are open.
So where's the Key ?
If you have come so far, you will stand in front of the Dark Coffer and have no idea of how to open it. The key is held by a Dark Keeper, which can be found inside the instance. However, he only spawns after you examine the picture which is located inside the Coffer Room (really hard to miss).
Dark Keeper Locations
Once you examine the picture a Dark Keeper will spawn at a location which is pointed out within the text. However, the hints given are not very helpful for people who don't know the Blackrock Depths that well, so here are the different locations of the possible Keeper spawns:
- This is the best spawn. Once you examine the picture he will patrol with his adds into the Coffer Room.
- Can be found near General Angerforge. You will need to open the Shadowforge Lock to get to him. After you pass the Shadowforge Gate go straight and after a few mob groups down the left stairs. He and his Bodyguards will be amidst another group of dwarves.
- Inside the Domicile, the area you passed not long ago. Go outside the coffer room, turn left, enter the Domicile, take the stairs in front of you to the upper level. Dark Keeper Pelver should be to your left.
- Go out ouf the Coffer Room and turn right. He will be inside the next room, which is basically a cave with various dwarves and golems.
- He is found inside the neutral Bar. You will need to open the Shadowforge Lock to get there. However, most people will rather reset the instance instead of clearing all the way to the bar, which can take quite some time.
- This one is found among the spectators inside the arena you passed on your way.
The Dark Coffer Loot
Dark Coffer Loot table http://wow.allakhazam.com/db/object.html?wobject=322
Once you have killed the Keeper, loot the Key and return to the Coffer Room to open the Dark Coffer and hopefully get even two Burning Essences.