The cosmic key was first used by Prometheus in his first confrontation with the justice league.
Is a key which can transport in and out from the "ghost zone", the place where Prometheus built his Crooked house.
Prometheus gain access to the cosmic key in one of his many travelers when a dark lama give him the chance to take the key. A key which literally could open an invisible door to another place. When the lama revelade himself as an inhuman creature, Prometheus killed him/it and took control of the key.
Prometheus also used the key to kill Retro.