Warning: this article deals with potentially disturbing subject matter.
Whatever the upcoming Flash movie will contain, it certainly won’t delve into the titular hero’s most horrifying and disgusting weakness. Even if the movie was rated R, it would probably be too extreme for Warner Bros to show this particular weapon on the big screen, but that didn’t stop one of the comics' most terrifying characters from threatening the Flash with it.
The Flash is one of the most powerful characters in DC Comics. His super-speed enables him to fight villains faster than they can react to him, and also allows him to do things like phase through solid objects and run on the surface of clouds. While he doesn’t have many traditional weaknesses like Superman’s Kryptonite, his powers have been able to be disrupted by extreme temperatures. Flash's speed has also been used against him by various supervillains. But none of these weaknesses are as memorable or disturbing as the one that the Robin King concocted to defeat him in the worst way possible.
In Dark Nights: Death Metal #3, written by Scott Snyder and drawn by Greg Capullo, the Robin King taunts the Flash before the Batman Who Laughs arrives to kill him. The alt-universe Bruce Wayne reveals that he has a way to kill every hero on Earth in his utility belt, including the Flash. He then reveals he dug up the Flash’s mother and coated her body in a Speed Force-nullifying toxin that liquefies the muscles of anyone who comes into contact with it. He then stored her corpse in one of the Flash rings that he usually uses to store his costume. If the Flash ever tried to attack him, the Robin King could trigger the ring, and Flash would be killed by the body of his own mother. The Robin King even states that when this happens it looks like she is trying to give her son a hug as her arms are extended outwards.
This is an incredibly disturbing but likely very effective tactic to defeat the Flash, despite the fact it is likely also a parody of other over-the-top deathtraps. The Flash has never truly gotten over the trauma of his mother’s death. He even caused a universal reset in his attempt to go back in time and prevent it. The weapon effectively weaponizes key elements of Barry's Flash in a needlessly cruel designed to break him both physically and mentally.
Dark Nights: Death Metal treated the Dark Multiverse as the home of "bad ideas" that were partly a parody of past 'grim and gritty' stories. In fact, what differentiates the Dark Multiverse from DC's main multiverse is that its worlds are half-formed and unable to survive for long. Despite this, Dark Nights: Death Metal reveled in the extremity of such single-use concepts rather than treating them with disdain. Snyder actually revealed that the Robin King was the maximum amount of horror he felt DC would let him get away with, explaining why his weapons are so disturbing.
Given even the comics didn't actually show this weapon, it's highly unlikely the movies would go so dark, but with the Dark Multiverse still an ongoing threat, it's possible Flash's nastiest weakness will one day return, turning the most tragic moment of his life against him.