In the last episode of season 2 titled “The Race of his life”, Zoom invited Flash for a race, the real purpose was to power the magnetar and create a huge trans-dimensional shock which will destroy the entire multi-verse. Flash finally agreed to race him, created a counter-vailing force to reverse the effects of massive energy generated to activate the cosmic device and finally let time wraiths took him as he distorted the rules of the speed force. But as they carried him through the portal, his body disintegrated and his physical appearance changed dramatically. His face turned eerily black which reminded us of Black Flash from the Geoff Johns comics.
Black Flash is literally the agent of death, as he carries speedsters and returns them to their source i.e speed force. In the comics, Wally west defeated him by racing to the end of time where death is meaningless. He is’nt really a villain, infact Barry Allen has taken up the mantle of Black Flash in the comics on several occasions.
The creators of the show Greg Berlanti and Andrew Kreisberg said to MTV “Everything about Zoom felt like death. We had an idea that it should look like a demon. Reverse-Flash came from the future and Zoom came from hell.” But now they want to go in for something sinister. He is either Bart Allen’s Reverse Flash (Edward Claris), Reverse Flash of New 52 (Daniel West), New Reverse Flash in DC Rebirth (Godspeed), Reverse Flash of Crime Syndicate (Johnny Quick), Impulse (part of the Thawne family) or the Black Racer (New Gods).
Well, he is Savitar from Geoff Johns original Flash Rebirth who became a thing during Mark Wade’s run. He started off like Max Mercury/Johnny Quick who got stuck in a speed force in the past and then he was brought into the present day. He is a Wally West era villain which explains the presence of Kid Flash. CHEERS!