Warning: SPOILERS for The Flash season 7, episode 4, "Central City Strong".
The Flash season 7 made mention of a fictional metal unique to the world of DC Comics that holds particular importance to the Legion of Superheroes. This, coupled with the mention of other fantastic metals across other Arrowverse series, has some fans wondering if the shows might be building towards something bigger.
The main plot of The Flash season 7, episode 4, "Central City Strong" concerned the return of Abra Kadabra; one of the supervillains profiled in Cisco Ramon's Who's Who Binder of Villains, who had been resurrected on Earth-Prime following Crisis on Infinite Earths. Originally a thief and murderer from the 64th Century of Earth-19, Abra Kadabra was also a time-traveler who used the advanced nanotechnology of his time to play at being a wizard, as he stole valuable treasures from the pasts of other worlds. Paradoxically, his first battle with Flash from Barry Allen's perspective was also their final battle from Abra Kadabra's perspective, and the mad magician returned to Earth-19 to face execution for his crimes, promising his past self would return again and again to become one of Barry Allen's greatest enemies.
Abra Kadabra finally made good on his threat in "Central City Strong," as he began teleporting around Central City and collecting a strange series of small yet super-heavy obelisks that weighed two tons each, despite being the size of a paperweight. Using his powers, Abra Kadabra combined the obelisks to create a sizable sphere and anti-matter magnet, slowly turning the sphere into an anti-matter bomb capable of destroying Central City. Cisco Ramon was finally able to identify the mystery metal used to create the obelisks as Valorium; an rare alloy primarily composed of the Nth Metal that empowers Hawkman and Hawkgirl. Valorium is also an important metal in the lore of the Legion of Superheroes, who exist in the 30th Century of the DC Comics multiverse and the Arrowverse.
Brainiac-5 used Valorium to serve as the core material in constructing the first Legion Flight Rings, which granted the power of flight to all those Legion members who could not fly under their own power. Little has been said about what differentiates Valorium from basic Nth Metal, beyond Valorium being denser and more durable. However, given that the Legion's rings also act as universal translators, personal communicators and life support systems that enable the wearer to survive in the vacuum of space, it is presumed that Valorium must have some ability to convey energy that powers the ring's other functions.
This mention of Valorium on The Flash is not the only example of an exotic metal from the comics taking a prominent role in an Arrowverse series in 2021. Recent episodes of Black Lightning revealed that a new anti-metahuman weapon, the Direct Energy Gun (DEG), was powered by Promethium; the same high-energy metal used to create Cyborg's implants and Deathstroke's armor in the comics. It remains to be seen if any of this will go on to be interconnected, particularly with the return of the Legion of Superheroes being teased for Supergirl season 6. Nevertheless, it is interesting to note that all of these metals featured prominently in a recent comic book storyline, Dark Nights Metal, which would serve as an interesting basis for an Arrowverse crossover.