Cobalt Blue (Malcolm Thawne)Created by Mark Waid, Brian Augustyn & Jim Aparo (Speed Force #1)
Did you know Barry Allen had an evil twin brother? You do now! On the night Barry and his brother were born, another woman’s child died on delivery due to the doctor's negligence. The doctor lied to Nora Allen and told her one of her twins didn’t survive, and presented that child to the Thawnes as their healthy baby boy to cover up for his mistake.
Malcolm Thawne grew up without the advantages Barry Allen had and was raised by con-artists who had the inherited power of a blue flame that could heal people, and Malcolm was mocked by his family for lacking this gift. He eventually discovered his origins, killed the doctor, and set about getting revenge on Barry Allen for stealing his life.
He discovered the power of the blue flame for himself, which was powered by his hatred for Barry, and became the villain Cobalt Blue. He was easily defeated by Barry Allen and Wally West, but resurfaced following Barry’s death, where Wally learned the truth of his origins and discovered that the hatred of the blue flame would carry on for one thousand years, inherited by members of the Thawne family for generations.
Over the course of a millennium, The Flash repeatedly fought Cobalt Blue in one form or another, with the power of the gem able to possess people at will, and eventually possessing an army of cross-time Flashes led by a possessed Jay Garrick. Wally and a time-travelling Barry Allen managed to stop Cobalt Blue, but at the cost of Barry’s life, which changed the timestream to one where The Anti-Monitor was victorious in Crisis on Infinite Earths.
Wally eventually changed everything back and defeated Cobalt Blue for good, although it cost him his existence as he merged with the Speed Force and was thought dead for some time, replaced by the Dark Flash, Walter West, of a divergent timestream.