Shrine of Lost Souls

The Shrine of Lost Souls is one of the wings of the Black Temple, hosting a great place of arcane power that Illidan used to feed his army of unbound demons in exchange for their service, freeing them from the addiction of fel magic. The warlock Kanrethad Ebonlocke later compared the Shrine's power to that of the Well of Eternity. He believes it to be the original power source that Illidan intended to offer the blood elves to free them from their addiction to the Sunwell, but for some reason he didn't let them near it.

The Burning Crusade
The Reliquary of Souls boss encounter is located at the end of this wing. It is composed of the Essence of Anger, the Essence of Desire, and the Essence of Suffering. Throughout the encounter, they release enslaved souls. The place is also filled with soul fragments.

Mists of Pandaria
Jubeka Shadowbreaker and Kanrethad Ebonlocke, two warlocks of the Council of the Black Harvest, infiltrated the Black Temple some time after the defeat of Deathwing. The two of them found the source of arcane magic beneath the Shrine of Lost Souls, which Kanrethad compared to the Well of Eternity. They sought to harness the magic of the place into an Empowered Soulcore crystal, but they were forced to leave before the crystal had charged as Ashtongue broken found them. The warlock adventurer later fought the Essence of Order and retrieved the Empowered Soulcore from it.

Mobs

 * Angered Soul Fragment
 * Hungering Soul Fragment
 * Suffering Soul Fragment

Origins

 * The Shrine is filled to the brim with lost and enslaved souls. It is possible that these are the souls of the dead draenei that Illidan siphoned from Auchindoun with a device, with the goal of powering a massive portal to Argus. However, he was killed before he could open the portal, instead he had to resort to sending his demon hunters to Mardum.
 * Alternatively, since its magic is compared to that of the Well of Eternity, it's possible that the Shrine was made from, or empowered with, the final vial containing the waters of the first Well. Illidan had originally filled seven vials. He used three to create a second Well of Eternity atop Mount Hyjal. One vial ended up in Dath'remar Sunstrider's hands, which he used to create the Sunwell. Two were given to his lieutenants Lady Vashj and Kael'thas Sunstrider. But the last and final one is still missing to this day.

Relationship with the elves
The reason Illidan never gave the blood elves access to this source of power (even though he originally intended to, according to Kanrethad), is still unknown.
 * It may be that he had suspicions concerning Kael'thas.
 * It's also possible that Kael'thas betrayed him before he could decide to give it to them.