Where can I find lore on the Heart of Darkness?

Question

On the General of Gehenna's Greyhawk wiki page I saw this

According to one legend, it was the General of Gehenna who created the tanar'ri and baatezu from the "impurities" within the yugoloth race using an gem-artifact known as the Heart of Darkness. According to the story, the law and chaos inherent within the yugoloths was projected into squirming larvae of the Gray Waste. The General spoke with the larvae, imprinting dark magic into their minds before having his minions herd them into the Abyss and Baator, where they evolved into the respective races of those planes.

Emphasis mine.

So anybody know where I can find lore on this cryptic artifact of old? 'Cause my Google Fu has failed me :(
it also kinda gives me Shard of Pure Evil/seed of evil vibes.

Answer 1

The only source that I have been able to find that goes into any depth about the Heart of Darkness is "A (More) Complete Guide to the Aligned Outer Planes", an AD&D (2e) Planescape third party handbook written by Dave King in 1999.

The information about the Heart of Darkness is on page 38, it goes into a good deal of detail and even has some game mechanics.

Take into account that it being a third party source, its expansion upon the official lore cannot be considered canon, so you would do well to not take it at face value.

If you're only interested in canon lore, your best bet is the "Hellbound: The Blood War" boxed set, also for AD&D 2e Planescape. On page 9 of the DM's Guide there is a page with some lore about the Heart of Darkness, though it isn't much, it's more than it's available in the greyhawk wiki.