Going back to the beginning of ‘The Flash’, it was known that the titular hero would vanish during the “The Crisis” (“on Infinite Earths”) and the show will follow the comics.

Showrunner Eric Wallace stated:

“‘Crisis’ turned out to be the best thing that could have happened to… The Flash because it created an immediacy to things.  We know that on December 10, 2019, the Flash will die…. We’re not messing around.”

DC Comics

What’s more, Barry (Grant Gustin) and Iris (Candice Patton) know about his fate, and the first part of Season 8 will show them reacting.

“As a married couple, they can count the number of days and weeks… What kind of urgency does that give them? An extreme one. It turns the dial up to 10 and it makes them think is every moment together our last? What can we do? Should we fight this? Is it inevitable? These are all of the things that they’re grappling with [until ‘Crisis’]. So their relationship is strained, but it’s also going to bring them closer together than ever before because that’s what tragedy does.

 

“The Monitor showing up and making a cosmic pronouncement drives you to extremes in order to fight against or accept the coming ‘Crisis.  And every episode this season… is all about ‘do I accept death or do I fight it?'”

In the comic book, Barry’s death was significant, because his Flash is considered the first superhero of the Silver Age and it was in his comic that the concept of parallel Earths was introduced.  He was captured at the beginning of the maxiseries and broke free about midway through, sacrificing his life to destroy the Anti-Monitor’s major weapon of destruction.

Obviously, something different has to happen on the TV show, because it really can’t go on without him.  But as has already been teased, Oliver (Stephen Amell, ‘Arrow’) is taking steps to change fate.  We’ll have to wait until the crossover to see just how things turn out.

“Crisis on Infinite Earths” kicks off on the Sunday, Dec. 8th episode of ‘Supergirl’ at 8 pm EST (special time), and continues on Monday, Dec. 9th on ‘Batwoman’ (special night), and Tuesday, Dec. 10 on ‘The Flash’.  It will conclude on ‘Arrow’ on Tuesday, Jan. 14 at 8pm (special time), and ‘Legends of Tomorrow’ on Tuesday, Jan. 14 at 9 pm (special episode).

 

Source: CBR