The producers of the Arrowverse having been hinting at it for a while now, and tonight it happened. During the final installment of 'Crisis on Earth-X', one of the heroes did indeed meet their demise. If you haven't seen tonight's episode, stop here because obviously, a pretty major SPOILER follows.
The hero whose run came to an end was none other than one half of Firestorm, Professor Martin Stein, played by Victor Garber. He was gunned down by Nazis on Earth-X while trying to reactivate a gateway home and save Earth-1's superheroes.
Stein first made his meta-appearance as one half of team Firestorm with Ronnie Raymond (Robbie Amell). He has since gone on to join with the Legends where he bonded with Jax (Franz Drameh).
Since the birth of his grandson, whom he named after his first Firestorm partner, Stein had mentioned wanting to retire in order to spend more time with him and his daughter. Jax and Ray Palmer had been working on a way to separate them without killing them, and it was ultimately Cisco who discovered a way to make that happen. The consequence was that it would strip both of them of their powers.
This solution that Cisco worked out was actually what killed Stein, when he took the serum in order to stop the toll his injuries were taking on Jax.
The CW's DC superhero series will return for their mid-season finales next week, beginning with Supergirl at 8 p.m. ET/PT and continuing with The Flash (Tuesday at 8), DC's Legends of Tomorrow (Tuesday at 9) and Arrow (Thursday at 9).