Black Mercy
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The Black Mercy was a telepathic, plant like species alien to Earth. 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.
[edit] History
As part of his plan to conquer Earth, Mongul sent one to Superman, as he believed the Man of Steel was the only real threat to his hopes to conquering the planet. Once attached, the Black Mercy fed Superman a reality where Krypton never exploded, and he was a simple farmer with a wife and son. Batman attempted to several different means to remove the plant, but was only able to succeed when Superman himself realized that the reality was fake. Unfortunately, the Black Mercy then attached itself to Batman, feeding him a reality where his father fought off the gunman that killed him and his wife. An injured Wonder Woman managed to get through to Batman and remove the plant, barely getting caught herself. Wonder Woman then threw the plant at Mongul, incapacitating him. His dream state is unknown, but it did not sound very pleasant.
[edit] Background information
In the DC Comics, the Black Mercy was used by Mongul as part of his revenge against Superman for ruining his chances to obtain Warworld. 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 that 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.
