A key was required to access the dungeon. The key was broken into multiple parts and carried by Shadow generals disguised as Undead creatures.
The dungeon that contained the shard could only be accessed by becoming Player Killer. Inside the dungeon two sides those protecting the crystal and those attacking would fight it out, Turbine devs also helped the attacking side by controlling Bael'Zharon's generals.
The above screenshot is of the Thistledown defenders before one of them defected and destroyed the shard.