Cathexis
The Cathexis were a pair of entities from the Sixth Dimension who created Id, a sentergy which could make desires into reality. Sending Id to three-dimensional space- Earth in particular- to test it, the sentergy latched onto Superman's subconscious desires and split six of the Justice League of America- Superman, Plastic Man, Green Lantern (Kyle Rayner), the Flash, Batman, and the Martian Manhunter- into their superheroic selves and their human alter egos. The result was an increasing and dangerous personality divergence in all twelve.
The Cathexis claimed Id escaped their realm by mistake, and asked the JLA to help recapture it. However, they simply used the JLA to find Id, so they could take control of the sentergy and use it to manipulate reality to their whims. The Cathexis were narrowly stopped when Eel O'Brien (Plastic Man's alter-ego) used one of their devices to partially remerge the alter egos with the JLA- a process brought to completion by Wonder Woman. The Cathexis were defeated when the Flash used Id to divide them into four three-dimensional beings, whereupon Batman soundly thrashed them. Directed by the JLA's combined will, Id sent the Cathexis back to the Sixth Dimension and sent itself into non-existence.
