Warning: this article contains spoilers for Future State: The Flash #2!
Make no mistake, the Flash is one of DC's most powerful heroes. The cosmic energy field that grants speedsters their amazing abilities, known simply as the Speed Force, does more than just allow people to move really, really fast - it allows them to travel through time, steal the kinetic energy from other moving objects, and (in the case of DC's Future State: The Flash) consume all of humanity. In a dystopian future spawned from the aftermath of Dark Nights: Death Metal, former Kid Flash Wally West is possessed by the spirit of Famine, who strips his old mentor of his connection to the Speed Force. Without his powers, Barry Allen arms himself with the technology of his former enemies, specifically a secret weapon that depowers speedsters known as the Still Force.
What is the Still Force? It's basically kryptonite for anyone connected to the Speed Force. Considered one of the "seven hidden forces," the Still Force was previously banished beyond the Source Wall until accidentally being freed into the greater Multiverse in the aftermath of Dark Nights: Metal. It is a cosmic energy field powered by entropy, meaning it can instantly negate the Speed Force. But its effects go far beyond speedsters - it can even pause the reality-bending imp Mr. Mxyzptlk in his 5th-Dimensional tracks. And beyond its cosmic-level stopping power, it can grant its user eternal life while also draining the life-force of others like a quantum vampire.
For Barry Allen to defeat Famine-possessed Wally West, he needed to build a combat suit similar to Batman's anti-Kryptonian armor. But instead of smashing Superman, the Flash's armored suit is equipped with the weapons of his Rogues, such as Trickster's flying shoes, Captain Cold's ice gun, and Mirror Master's duplicator tech. But his ultimate secret weapon against the Speedster of the Apocalypse once belonged to a rather odd villain known as the Turtle. Once a scientist determined to prove the existence of a cosmic force governing inertia, the Turtle's experiments transformed him into the polar opposite of the Flash.
While only a minor villain for most of his tenure as a Rogue, everything changed once the Still Force was unleashed from beyond the Source Wall. In Scott Snyder's run of Justice League, the elderly Turtle is reborn as a baby and teams up with fellow Flash villain Gorilla Grodd. Baby Turtle's connection to the Still Force combined with Grodd's telepathy proved to be too powerful for the League to handle. At one point, Turtle and Gorilla are even able to halt the expansion of the universe.
So, why is Future State's nod to the Still Force so important? It means that despite the omniverse having been repaired, the Still Force is no longer trapped behind the Source Wall, meaning this incredible is now a part of DC's ongoing canon and the Flash's future adventures. With the Flash's power level constantly increasing like that of a Dragon Ball character, it's important that he have some ultimate weakness to even the playing field in certain situations. Will the Still Force reach the same level of infamy as kryptonite? Probably not, but it will no doubt serve as an "Achilles heel" to the Scarlet Speedster for decades to come.