The Flash has just revealed another of Barry Allen's weaknesses, ahead of the "Crisis on Infinite Earths" crossover. The entire Arrowverse is building towards a Multiversal crisis, the biggest crossover in the history of DCTV. And, as revealed by the Monitor, Barry Allen is fated to die so that his entire world can live.
Enter Nash Wells, a Multiversal explorer who's arrived on Earth-1 seeking out the Monitor. He believes that the Monitor himself is responsible for creating fear on countless Earths, and intends to save the Flash by defeating the Monitor once and for all. Nash has discovered a mysterious lair hidden beneath Central City, with an impenetrable barrier protecting a portal to the Monitor's own realm. Of course, viewers know that the situation is more complex than Nash Wells believes, and that he may well be on the trail of the Anti-Monitor - a being who he really doesn't want to go up against.
The Flash season 6, episode 6 opens with Team Flash attempting to help Nash get through the barrier. Headstrong as ever, Barry Allen figures he can just phase through the wall in the sewers beneath Central City, but he's shocked when he's blasted back. According to Nash, the problem is that the barrier is saturated with Eternium, and that frankly Barry's lucky a blast of energy wasn't released that killed them all. It seems that Eternium is entirely impenetrable - and can't be phased through.
In the comics, Eternium is a mysterious, magically-charged substance that originates from the Rock of Eternity. Viewers will be familiar with the Rock of Eternity as the home of the wizard in Shazam!, but in the comics it's the source of all magic, and sits at the very heart of the Multiverse. When it was destroyed in 2005's Day of Vengeance miniseries, the mineral Eternium was scattered across the Multiverse. The Arrowverse origin of Eternium is uncertain, but it seems to be associated with both the Monitor and the Anti-Monitor.
If Eternium is truly so powerful, and indeed is resistant to Barry Allen's phasing power and super-speed as the latest episode of The Flash suggests, then it bodes ill for the heroes of the Arrowverse. They're soon going to be plunged into the Crisis on Infinite Earths, caught up in the cosmic battle between the Monitor and the Anti-Monitor, with the fate of the entire Multiverse at stake. And it seems that these two beings liberally use a mineral that is proof against one of their greatest advantages, the Flash's speed.