The Negative Speed Force was created by Eobard Thawne as a negative version of the Speed Force. It is an extra-dimensional field of negative energy that eats away at the regular Speed Force like a cancer and can consume any speedster connected to it by touch. Like Barry Allen with the Speed Force, Thawne himself is also the generator of the Negative Speed Force with every step that he runs. He used it to infect and corrupt Barry, in the hopes that the Negative Speed Force would consume Barry's loved ones and other speedsters who rely on it, consuming Barry in grief so that he'd merge with the Speed Force.
In the Post-Flashpoint universe, and after the events of DC Rebirth, it is revealed that the Negative Speed Force still exists and that Eobard Thawne is still the creator and generator of it. Thawne can tap into it as normal speedsters can with the normal Speed Force, but with its own unique energy signature. The energies have turned from red to black to further emphasize its "negative" and poisonous properties.
Thawne leads Barry to the Negative Speed Force, as he figures that it will keep him trapped there forever. Instead, Barry's Speed Force connection is corrupted, now gaining a Negative Speed Force connection as well as a different look. Barry somehow escapes and returns to the 25th century, and promises he'll kill Thawne.
After Iris West killed Thawne, the Negative Speed Force seemed to remain within Barry's system causing him to accidentally produce destructive force waves around him, blowing up an armored truck after saving its drivers from the Road Reapers. Barry soon realizes his accelerated metabolism has diminished since his exposure and takes longer to heal from superficial wounds such as those received from his battle with Shrapnel. He later, unfortunately, learns the Negative Speed Force is also playing havoc with his emotions, causing him to snap at David Singh and the destructive aura surrounding him is exacerbated by anger. Wally West later volunteers to train Barry to control his new destructive abilities. 
It would appear the Negative Speed Force can be kept in check to an extent by its opposite, the positive Speed Force as Barry and Wally stop a jewelry store robbery by Black Spider. Barry's aura remains quiet when he runs alongside his partner.
A few days later, Meena Dhawan returns and tells Barry that the negative aspect of the Speed Force is conflicting with the remnants of the positive in his system and is slowly killing him. She orders him to test the full destructive power of its energies at a Demolition Derby, the results shattering windows and flipping nearby cars. But Meena later betrays Barry and Wally, the full force of Black Hole appearing and capturing the speedsters, Meena seemingly as their leader. She then proceeds to circle him at super speed and absorbs the Negative Speed Force into herself. 
Those with access to the Speed Force can use a large number of abilities, usually revolving around speed. The Negative Speed Force grants the users all of the abilities that the positive Speed Force does, such as the power to move at vast superhuman speeds, as well as superhuman physical characteristics, regenerative healing, enhanced mental capabilities and perception, lightning generation, molecular control, and the power to traverse throughout both time and dimensions.
Some abilities the Negative Speed Force grants that the positive Speed Force does not are: the ability to negate Speed Force users' powers, the power to absorb memories from others, and the power to alter one's age at will, as well as tearing out Negative Speed Force from others without killing them. Also, Negative Speed Force users have the ability to accurately time travel without devices such as the Cosmic Treadmill very easily, not having to run at the Speed of Light, a perk from the Negative Speed Force, along with entering the Speed Force at a similar speed.
Negative Speed Force conduits generate large amounts of electrical energy that they draw from the Negative Speed Force. The color of the lightning varies from red, black, and purple. Unlike positive Speed Force lightning, Negative Speed Force lightning does not act like normal electricity -- It is far more destructive and can cause objects to explode on contact.
The protective aura manifested by the Negative Speed Force also is far more destructive than the normal Speed Force's aura. It can cause objects such as bullets to incinerate before they can hit the speedster. The aura also builds up every time the speedster is dealt damage and will eventually release an explosion.
Should a natural Speed Force user be exposed to the Negative Speed Force, it will cause great pain to them and will sometimes cancel certain positive and negative speed force powers while trying to use both at once.
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