Black Mercies

"The Black Mercies are a telepathic species. It reads the heart's desires and feeds the individuals a totally convincing simulation of it."

- Mongul about the Black Mercy plants.

The Black Mercies (also sometimes Black Mercy in plural) are a telepathic and parastitic plant-like alien species. It was capable of attaching itself to a host, read the host's memories, then feed the host a totally convincing simulated reality that was based on the host's deepest personal desires. What the Black Mercy received in return for this state is unknown.

Origin
Also known as Ater Clemtia, the Black Mercy were a species of parasitic plants that were native to a world in Space Sector 2811. Their origins were traced to being the empathic offspring of the living planet known as Mother Mercy and were seeded by her upon maturity whereupon they were sent into the stars on a mission to alleviate the pain and suffering felt by the sick or by the dying.

Mongul
At some point, the tyrant Mongul I discovered the home planet of the Black Mercy plants whereupon he began harvesting them for a more sinister purpose. Twisting the creatures, he intended for the plants to be a killing tool and thus distorted their true original purpose by making them infect healthy hosts whereupon they slowly began destroying their victims. 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 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.

Sinestro Corps
After the Sinestro Corps War, Mongul II discovered a Qwardian Power Ring whereupon he became a powerful member within the leaderless Sinestro Corps. Using his new power ring, he genetically altered a number of Black Mercy plants which transformed them from weaving dreams within their hosts to making them instilling great fear on their victims. 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.

Powers and Abilities

 * Parasiticism : it had the capacity of feeding of the bioelectrical aura of a host organism until it died. It accomplished this feat by attaching to its prey with the main roots that consisted of needle-like vines which entered the skin of the host whereupon it fused with the nervous system. They left the host in a state of euphoria whereupon the plants slowly killed the host as it fed from the feelings generated by its victims.
 * Dream Worlds: after infecting a host and wrapping their tendrils onto them, the plants began altering the visual perceptions of their victim leaving them in a dream-like state where their fondest desires as well as wishes came true. This left them 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.
 * Nightmares : when they were manipulated by a Qwardian power ring which distorted their natural talents, the plants were changed with a new ability of making their hosts live in a nightmare state where their deepest as well as darkest fears came into reality.

Dream Worlds
The Black Mercy plants are capable of creating "dream worlds", each faux utopian realities that are a complete and utterly convincing simulation of the victims' deepest desires, such as wanting to very rich and famous, longing for a family or becoming either a hero or a ruler. There are known victims whose "perfect worlds" are the taste of the psychic and parastitic powers of the Black Mercies.
 * Superman: in this simulated alternate reality, his father Jor-El's prediction was proven untrue. He is married to Loana, a beautiful Kryptonian woman who is a composite of Lois Lane and Lana Lang, and together they has two children including their son Van-El. In the midst of this, Krypton is going into politcal upheaval. His cousin Supergirl (who would be almost 16 years older than him now) is attacked by anti-Phantom Zone protesters.
 * Batman: In this dream, young Bruce Wayne's parents' murder is foiled. He grows up and has a wife named Kathy Kane, and they both have a daughter.
 * Mongul: In this nightmarish dream, Mongu himselfl kills the people he is fighting -- Wonder Woman, Superman, Robin, and Batman -- and goes on to conquer the entire cosmos.
 * Green Lantern: In this dream, Hal Jordan's father never died nor did his mother. Coast City was never destroyed. Sinestro never turned evil, and is his mentor and friend. They show the Green Lantern Corps fighting an unknown man possessed by Parallax. His family is together happy and he is dating his high school sweetheart Jen.
 * Green Arrow: in this dream, Ollie is married to Sandra Hawke and has a great relationship with his son Conner. He has at least three kids. His son Connor Hawke was Speedy or the equavilant of it. His identity is publicly known.

Flaw
The worlds are temporary and people always break free. Either the plants lose their grip, or the people under its thrall may realize it isn't their lives or life is never this perfect. It is possible it doesn't work because desires are empty.

Trivia

 * The Black Mercy plant appears in the Justice League Unlimited episode "For The Man Who Has Everything", which is based on the Superman Annual volume 1 story of the same name.
 * In the DC Comics, the Black Mercy was used by Mongul as part of his revenge against Superman for ruining his chances to obtain War World. Following the Crisis on Infinite Earths, the Black Mercy plant was retconned to be a symbiotic species created by Mother Mercy, a sentient plant organism, and were intended to latch to terminally ill sentients and give them a happy, painless death. Mongul's son, Mongul the Lesser, later used several in a plan to kill his sister Mongal.