Originally Posted by
Londo Bellian
BLACK MERCY: an alien plant from DC Comics that can subject a sapient life-form into a coma state where they dream an ideally perfect mindscape of their lives, drawing their deepest desire and wishes to manifest a perfect existence in the mind. The Black Mercy was introduced in 1985 on "Superman Annual #11: For the Man Who Has Everything" by Alan Moore, where the alien warlord Mongul ambushes Superman in his fortress and traps him in a Black Mercy dream. Luckily Batman, Robin and Wonder Woman have dropped by because it was Clark's birthday. Cue epic battle while Superman dreams he was still Kal-El living on an unexploded Krypton, with a family.
That was HIS Black Mercy dream, though as he begins seeing through the cracks the peaceful Krypton devolves into a conflict zone until he awakes when Batman removes the BM from him. Then the plant latches onto Bruce, subjecting HIM to a dream where his father Thomas beats up Joe Chill, allowing the senior Waynes to live all through Bruce's peaceful adulthood. Robin removes the BM from Batman and uses it on Mongul, where HIS dream sees him "evade" the Mercy being thrown at him, killing Superman, Batman, Robin and Wonder Woman in quick succession, and moving on to conquer the universe.
On the "Justice League Unlimited" adaptation of "For the Man Who Has Everything," only Batman and WW visit Superman that day. The political crisis dream-Krypton undergoes as Kal-El starts fighting the Black Mercy's creation is instead replaced with the seismic upheaval that canonically destroyed it playing out the more Superman becomes cognizant of his situation.
The Black Mercy species will reappear in other DC Comics titles since. On TV, it also appeared on The CW's "Supergirl" with Kara being the victim, and reinterpreted as growing stronger and more attached to its host the longer it remains, instead of the opposite.
And there's your Black Mercy recap. Can you visualize how a Krakoan will dream under its influence now?