The Dark Coffer 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.