The Fort Rozz escapees are the former inmates and escapees of Fort Rozz.
Fort Rozz was located in the Phantom Zone, but when Kara Danvers' pod went to the zone, it brought Fort Rozz to Earth and consequently freed the inmates. Astra, the leader of the escapees soon desired to become Earth's leader and to begin with she wanted to kill Supergirl. Astra sent Vartox to kill Supergirl, but the latter was killed instead by the heroine.
The escapees soon focused on killing Supergirl, Astra and Gor confronted the Hellgrammite and forced him to help them again. Hellgrammite captured Alex Danvers, to attract Supergirl. Astra then confronted Supergirl and fought her, almost killing her. Alex then killed the Hellgrammite and the D.E.O. forces arrived. Astra then attempted to kill Hank Henshaw, but was stabbed in the arm by a knife made of Kryptonite.
Non took Hank Henshaw hostage and used a former psychic Fort Rozz escapee to learn the location of Astra. The alien fails to probe Hank's mind due to his own powerful psychic skill. Non killed the mind reading alien on the spot due to his failure.
The Master Jailer was a guard at Fort Rozz, he masqueraded as Carl Draper, a National City police detective, while hunting down and killing the Fort Rozz escapees who were disguising themselves as Earthlings. He killed Prisoners #2440 through #2444. However Supergirl was able to save Prisoner #2445 and allowed him to return to his human life.
Eventually all Fort Rozz escapees were defeated or arrested.