You must have multiple players in order to complete the dungeon. There are multiple reasons for this:
You have to deal 50,000 damage within 5 seconds to the wall at the start room. This requires more than one character (unless you use a maxed attack and dexterity warrior with Demon Blade, the berserk buff and a few dexterity tinctures)
Jon Bilgewater, the boss, is healed periodically. You need to deal enough damage in order to not only counter the heals, but exceed them.
Although this dungeon is very difficult, it is one of the few sources for both speed and dexterity potions, as well as the highest tiered equipment. There is also a chance of finding a rare untiered sword.
The dungeon is a guaranteed drop from the Crystal Prisoner, but can also be purchased in the nexus for 390 gold (price varies.). You are also able to obtain the key through various contests as announced by Kabam.