Eva Belyakov was an Inhuman with the ability of extraordinary strength. She was the inhuman behind Melinda's mission in Bahrain, the mission that earned the name "The Cavalry".
While Eva was at Afterlife, she desperately wanted her daughter, Katya, to go through Terrigenesis and unlock her Inhuman potential, claiming it was her daughter's birthright. However, Jiaying saw a darkness inside Katya's heart and didn't want her to go through the mist.
Not trusting Jiaying's judgement, Eva stole a batch of Terrigen Crystals and had Katya go through Terrigenesis without Jiaying's training before hand. The mist gave Katya the power to feed off of the emotions of others like a psychic parasite and the power to control others. Katya was too young to control her powers, however, and she quickly became insane.
Katya brainwashed her mother and a gang of thugs in Bahrain and was feeding off of all the pain they were feeling.
A S.H.I.E.L.D. team was sent into to stop Katya, but she quickly brainwashed all of them into obeying her.
Coulson sent in May next who started fighting with Eva. After May killed Eva, Katya tried brainwashing May saying she need a new "mother" now. At the last moment, May shot Katya and with her dead everyone under her control was back to normal, but with no memory of what happened.