Mnemon is a DC comics supervillain and primarily an enemy of the Justice League of America - like many JLA villains he is considerably more powerful and grand than a standard supervillain and can be considered as a threat to entire superhero communities rather than a single hero's "rogue gallery".
Mnemon itself began as a synthetic life-form created by an unknown being that believed the purest music was that of memories, thus Mnemon was designed specifically to collect memories.
However Mnemon soon developed the typical qualities of a supervillain, such as power-lust and greed - to the point it went insane and stole the memories of its creator and his entire world before destroying said world.
The crazed Mnemon then became a cosmic threat, travelling from world to world stealing the memories of entire civilizations before destroying their worlds and wiping out their species - soon the evil Mnemon sought out Earth and his battle with the JLA began..
- the idea of Mnemon is similar to Brainiac, another DC villain who also frequently destroys worlds after collecting vast amounts of information on the world in question : both are also rogue A.I. originally designed for benign purposes but willingly defied these origins in favor of megalomania (although Brainiac's original origins were that he was an organic alien, recent retcons gave us the more known A.I. version of Brainiac fans tend to associate with the character).