Once long ago, there was 5 Elementals, one for each element, a Light Elemental, A Dark Elemental, Water Elemental, Fire, and Earth/Wind Elemental. Dark was in charge of the nighttime, dreams, and nightmares, Light was in charge of the day, Water was in charge of the oceans, rivers and lakes, Fire was in charge of flames, and Earth/Wind was in charge of the weather. Dark soon found that people hid away and ran whenever him or his night came around, while all the other Elementals were worshipped. He grew bitter and jealous, and rebelled. He used his powers for worse, causing everyone to have nightmares and he tried to make everything pitch black from the nighttime. Dark had been one of the strongest of all of them, and it took the other Elementals a lot to stop him, in fact they couldn't, but they were able to just weaken his power so he could not wreck the balance between them any longer. Later, Light fell in love with a mortal and had two children, both girls, the slighty older one, Lilith, turned out to be a sixth Elemental, the Elemental of Neutrality and Peace. Later, Lilith grew up, and started a new chapter of an old story.
A book I might write, an Elemental is basically an immortal being rep. a certain element. Also i know this sounds a lot like MLP with Luna and Celestia, but trust me, it's not.