The Null Field is a device created by Bernadeth to nullify the powers of a prisioner.
History
After opening a portal to Apokolips to rescue her friend Twilight [1], Supergirl was eventually captured by the Female Furies and trapped inside a Null Field,and put in a closet where it would be another one of Granny Goodness's toys.
Capabilities
The Null Field is capable to nullify any kind of power which includes Supergirl's Earth-Born Angel powers . Composed of an unknown material, the device is unbreakable both inside and out, but at the same time it's malleable enough for Bernadeth to put Fahren-Knife inside and cut Supergirl's face. [2].