The Demon Seed
The Demon Seed is a glowing red crystal surrounded by flames. It is a powerful fel artifact valued and protected by the Burning Blade. It can be found at Dreadmist Den in the Northern Barrens.[42, 38] It's guarded by Xelnaz the felguard.
Background
The Demon Seed is a powerful tool for the Burning Legion, guarded by Xelnaz. It rests in a cave on Dreadmist Peak, above a demonic altar of fire. The Burning Blade rallies around this relic. For reasons unknown, Neeru Fireblade wished to destroy this artifact, although it was a valuable asset of the Shadow Council. It appears that it was never destroyed and now Balgor Whipshank wants it taken to his sister Nagala Whipshank. It's unknown what she plans to do with it.
Objective of
Looting 
[Demon Seed] is the objective of 
[5-30] Demon Seed.
In Hearthstone
In Hearthstone's Book of Mercenaries story, the Demon Seed is an important plot point as the reason that Tamsin Roame, the mad Forsaken warlock, travels with the other mercenaries in the Barrens. Desperate for revenge against the Kingdom of Stormwind for not coming to Lordaeron's defense during the Third War, she takes the Demon Seed and uses it in a ritual to summon Anetheron and the Burning Legion within the walls of Stormwind.
Patch changes
Patch 4.1.0 (2011-04-26): Now contains [Demon Seed], instead of being destroyed for the quest [14] The Demon Seed.