- Not to be confused with the Dark Heart.
The Heart of Darkness was a mystical crystal that enclosed the spirits of an evil race of snake people — the Ophidians.
Eons ago, early humans and the Ophidians waged war for control of the Earth. Over the years, the Ophidians were beaten to the point of extinction. Hoping to find a way to strike back, a group of warriors performed a ritual which placed their souls in the Heart of Darkness. Fearing what would come, ancient man built a temple around the site of the ritual, which was to be guarded for all time.
Whoever held the crystal would be compelled to destroy all of humanity by any means possible. Individuals influenced by the Heart of Darkness could be identified by their behavior: their constant search for ways to destroy humanity while humming the chant the Ophidians sang during the ritual.
There were only two ways to break the spell: show the influenced person intense light, or decapitate the host. The Heart's victims showed no memory of their actions under the artifact's control.
In the 21th century, human soldiers found the Heart of Darkness and overpowered Mophir, the last of the guardians of the temple. Burns, who had taken the crystal, learned of nuclear weapons and tried to use them, but failed. However, he managed to give the crystal to General McCormick, who had just overseen development of the AFD. Learning of the Justice League, the possessed McCormick drew them to a power plant, where he managed to give the Heart to Wonder Woman. She stole the AFD and smuggled it aboard the Watchtower, handed the Heart to J'onn J'onzz, who unloaded the device before giving the crystal to John Stewart.
Having learned of the Heart from Mophir, Flash began using a light-inducing gemstone Mophir had given him on the League, but the possessed John destroyed it. He was subdued and dropped the crystal, but Hawkgirl shattered it with her mace, causing it to explode into shards. The shards became embedded within the skin of all the Justice Leaguers - save the Flash, who was fast enough to avoid the shards.
In addition to hunting the uninfected Flash, the possessed League members sent the AFD into the Sun, which would destroy it and doom human kind. However, Flash was able to drive the shards out of the Leaguers with a searchlight, and the Justice League subsequently created a wormhole which drained the damaged matter away from the sun.
What happened to the shards is unknown, but they were likely collected and stored away by the League, in order to prevent the Ophidians' legacy from ever resurfacing.
The Heart of Darkness was a crystal from Apokolips designed to seal the Angel of Wrath. It was later shattered and unleashed the Angel, known as Eclipso to the Justice League. Several people have been possessed by Eclipso, including Bruce Gordon (the original Eclipso) Jean Loring (ex-wife of the Atom) and Mary Marvel (sister of Captain Marvel). However, Eclipso can only function during the night, as sunlight suppressed it.