"Pain and suffering are all that await you!"
The Reliquary of Souls
was a container of souls found within the Shrine of Lost Souls deep within the Black Temple. It's origins, and true purpose, are unknown. It's possible that it may have once been a draenic artifact, perhaps intended to serve a similar purpose as Auchindoun: to keep draenei souls safe and protected from the Burning Legion, only to have been corrupted and twisted by the Shadow Council when the Orcish Horde conquered Karabor. Regardless of its origins, the Reliquary currently contains and shepherds a host of souls, as the Shrine of Lost Souls lives up to its name. These souls, while they seem to be used to empower it, would also rise to the Reliquary's defence should need be.
The Reliquary was driven by three Essences: Suffering, Desire, and Anger, possibly reflecting the nature of the souls it contained. Each Essence had powers of their own to defend themselves, and would emerge once the previous Essence had been defeated. Despite their best efforts, each Essence was defeated and the Reliquary was destroyed.
"Face the souls of those your kind doomed to perish, Warlock!"
From the remains of the Reliquary, a fourth Essence emerged: the Essence of Order. It was encountered by warlocks searching for the secrets of Green Fire. The Essence of Order was focused on defending the Shrine, and the souls and power it contained.
These warlocks also discovered that the Shrine of Lost Souls contained a vast reservoir of pure, arcane power, capable of relieving anyone who drew on it of their own addiction to magic. Due to its similarity to the Well of Eternity, it is believed that this font of power was created by Illidan, who intended to use it to hold up his end of the bargain with Prince Kael'thas
and cure the blood elves of their addiction to magic. However, for whatever reason, Illidan withheld this from Kael'thas and the blood elves, and instead used it to bribe a host of demons into serving him.
Warlocks defeated the Essence of Order, and claimed the power of the Shrine of Lost Souls for themselves, storing as much of it as they could into an empowered soulcore. They couldn't stay for long, however, as in its death, the Essence of Order unleashed a horde of demons it had been previously holding at bay.