The Black Mercy, also known as Ater Clemtia, is a parasitic plant species that, upon attaching itself to an individual, alters the visual perceptions of a victim leaving them in a dream-like state where their fondest desires as well as wishes came true by tapping into the pleasure centers of the brain. This left the host trapped within their own bodies which were in a near vegetative state whilst the plants began to slowly feed off their dreams until the hosts died.
The plants had the capacity of feeding of the bioelectrical aura of a host organism. They accomplished this feat by attaching to its prey with their main roots which consisted of needle-like vines which entered the skin of the host whereupon it fused with the nervous system.
The host is left in a state of euphoria whereupon the plants slowly kills the host as it feeds from the feelings generated by its victims.
The plant can lose its grip on its host if the individual starts noticing that the reality is fake.
The Black Mercy plants are the empathic offspring of an alien plant-being known as Mother Mercy.
Originally living at the bottom of the sea on the planet Matris Ater Clementia in Space Sector 2811, when a mortally wounded alien fell from the skies she rose to the surface and found she was able to tap into the alien's center of thought and transmit personal images of pleasure and desires which in turn would release neuro-dampeners in the alien's body, allowing it to die in peace.
After the alien's death Matris could feel the pain of others emanating across the void of space. Planting herself into the earth she spawned spores that would become known as the Black Mercy, which would leave their native planet in a quest of easing the suffering of others without hope. To feed the cycle of new spores, sustenance needed to be found; to solve this dilemma the dead were brought back to the planet to nourish it with their flesh and blood.
At some point, the tyrant Mongul discovered the home planet of the Black Mercy plants and began harvesting them. Twisting the creatures, he intended for the plants to be a killing tool and thus distorted their original purpose by making them infect healthy hosts.
At the time of his harvest, Mother Mercy decided not to reveal her true nature and simply allowed the supervillain to continue upon his goal. He used the plants in his battles against Superman and the Justice League of America until he stopped coming as a result of his death. However, in time, his son came to the world and continued his father's usage of the Black Mercy plants.
Mongul II later travelled to the planet Earth in order to accomplish his deceased fathers wishes of conquering that world. With him, he brought a number of Black Mercy plants which infected animals and beings in a farm on the planet. Hal Jordan and Green Arrow after arriving in the field discovered a large number of Black Mercy plants which had infected the cows in the region. Mongul II ensured that the plants had plenty of water and "food" by allowing them to infest the native Humans in the region. During a fight with Mongul II, Hal Jordan attempted to burn the Black Mercy plants but during the skirmish both him and Green Arrow where infected by the creatures who made them live in a dream-like state.
After the Sinestro Corps War, Mongul II discovered a Qwardian Power Ring. Using his new power ring, he genetically altered a number of Black Mercy plants with a new ability of making their hosts live in a nightmare state where their deepest as well as darkest fears came into reality. They were used against a number of members of the Green Lantern Corps which were investigating Mongul II's operations, where they infected Sodam Yat and Arisia, before Mother Mercy revealed herself to the Green Lanterns who came to rescue them, attempting to stop them from burning her children. Upon being injured by Sodam Yat, she began to telepathically grieve and sob for her children, prompting Soranik Natu and Kyle Rayner to stop their fellow Lanterns killing her.
Following the defeat of Mongul II, Mother Mercy was recruited for both the Sinestro Corps and Green Lantern Corps. She ultimately chose to serve the Green Lantern Corps and is now the Green Lantern for her sector; Sector 2811.
Within the dreams induced by the plants, the image of the Black Mercy can usually be found in various forms.