Atomos is a boss and summon featured in the Final Fantasy series. He originated in Final Fantasy V as a boss ouside of Exdeath's Castle. In this video game, he is known as one of the more challenging bosses, which caused him to become very popular among Final Fantasy players - so much so that he eventually became a summon (starting with the PlayStation exclusive Final Fantasy IX). His first Nintendo system appearance as a summon is Final Fantasy XII: Revenant Wings for the Nintendo DS, where his appearance is dramatically different from before.
It should also be noted that Atomos was included as a boss in the Game Boy Advance remake of the two original Final Fantasy games. In this game, the battle is very similar to the one featured in Final Fantasy V.