A new trailer for the upcoming crossover event Elseworlds has revealed that the Earth of the 1990 The Flash series has been destroyed by the same forces that will be rewriting reality in the Arrowverse. This may signal the first steps towards The CW attempting to recreate the events of the classic comic book storyline Crisis on Infinite Earths - an event that was foreshadowed in the first episode of the current The Flash television series.
The Arrowverse Flash series has paid tribute to the 1990s Flash show in several ways over its first four seasons, mainly by casting actors from the earlier series in the new one. John Wesley Shipp (the '90s Barry Allen) plays both Barry's father, Dr. Henry Allen, and Jay Garrick - The Flash of Earth 3. Mark Hamill has also appeared several times on the show, recreating the role of James Jesse (aka The Trickster) as well as James Jesse's Earth 3 counterpart. The new series also established that the older series was part of the Arrowverse's multiverse in the season 2 episode "Welcome To Earth 2" when the 1990 Barry Allen was seen among the visions of other Earths, including Earth-38 - the home of Supergirl.
The latest trailer for Elseworlds, which can be viewed below, shows the the John Wesley Shipp version of Barry Allen standing alongside Supergirl and a Barry Allen and Oliver Queen who have had their superhero identities reversed. At the 0:13 mark, the older Barry Allen can be seen to angrily declare "Enough! You will not do to this Earth what you did to mine!"
While it is unclear who The Flash is addressing in the trailer, it seems likely that he is facing the Monitor Mar Novu. A race of all-powerful cosmic beings charged with maintaining the continuity of all realities, The Monitors in DC Comics are generally neutral in executing their duties, only acting to destroy those beings whose actions threaten the cohesion of the universe. There have, however, been instances of Monitors going rogue, and that seems to be what is occurring in Elseworlds.
UPDATE (This paragraph contains SPOILERS from Supergirl):
This seems to have been confirmed as of Supergirl season 4, episode 8. The episode ends with a scene set on Earth-90, depicting the aftermath of a great battle between many superheroes and an unknown enemy. The John Wesley Shipp version of The Flash is confronted by one of The Monitors, who say the heroes failed and now must be destroyed. The scene ends with this version of Barry Allen running toward the camera into a close-up on his chest emblem, as the theme from the classic 1990 The Flash series plays.
It remains to be seen what will become of the Barry Allen from the 1990s The Flash series. Will his world be restored to its proper place after he aids the heroes of the Arrowverse with saving their Earths? Will be find another Earth in the multiverse in need of a hero? Or will he, much like The Flash in Crisis on Infinite Earths, be forced to make the ultimate sacrifice to save all reality? One thing is for certain - come what may, the Arrowverse will never be quite the same after Elseworlds is over.