The Ater Clementia or the Black Mercy as it is known to be called is a parasitic alien plant that latches onto a host feeding it's barbs directly into the nervous system of intended victims to induce and enhance the pleasure centers of the brain, simulating for the victim an ideal paradise where in that world ones true hearts desire is made real.
Dreams simulated by the mercy are so potent that while an individual can shake off the beautiful vision it provides they choose not to. Thusly enabling the plant to feast upon they're body, sucking them dry until eventual and inevitable death.
They're creator the Matris Ater Clementia is an Omnipathic vegetative lifeform who could stimulate the pleasure and horror senses of individuals she symbiotically merged with and had initially created the mercy's as a means to ease universal pain and agony by spawning and seeding her offspring throughout the universe. However a cosmic dictator by the name of Mongul; later his son, found a more military usage for the fantasy inducing foliage.
It's most infamous usage was during the titular story line "For the Man who has Everything", where the tyrannic despot sent it disguised as a gift to the last son of krypton to remove his interference in his invasion of earth. As well as a cruel method of revenge where the man of steel had to sacrifice his most cherished desire to ever return to the waking world again.
Numerous times the Mercy's Effects have been altered to suit the need of individual Monguls. The first time around was when the Son of said Conqueror had altered them to induce ones worst fears instead of their most cherished desires, forcing the individual to experience what they deeply dread.
This was accomplished by infusing the plant with yellow fear energy from a Sinestro Corps Ring to alter they're functions on a biogenic level. Within the DCnU The Black Mercy remains remains relatively unchanged; save the reinvented Mongul has changed them to first calm hosting beings with soothing dreams of their hearts content before plunging them into a hellish mirage of inescapable horrors generated from their Id.