Savitar

Nothing is known about this unnamed individual, except that they have a metal arm, and are affiliated with Alchemy.

Biography
After the Rival failed, Alchemy scolded him and then this individual entered his cell at Iron Heights in the meta-human wing and attacked him. Upon investigation, Barry, Joe and Julian discover that this individual killed the Rival the night he was incarcerated using powers of some kind. Later, the rest of Team Flash are all informed of this, and Harry guesses that maybe this individual was using telekinesis or cloaking abilities.

Powers
It's currently unknown what exact power(s) this individual possesses, but they are obviously very powerful and were able to enter into the Rival's meta-human cell, that until now, no with powers has been able to escape, and killed him before he hit the ground.