When night falls, we find peace in the knowledge that daytime will return soon, and vice versa. But what if the world was split: daytime and night are two different, coexistent worlds, each with it's own laws and rules. At dawn and sunset, our soul switches to inhabit our other being, our 'twin'.
MoonInk: This alchemical product has 101 uses. The Ink/Paint dries to be totally clear and almost unnoticable. Under moonlight, it comes forth as a silver ink. It was originally used by the Elventi to enhance certain pieces of art. It is used by espionage types to produce secret messages, nobles for hiddle plots, and diabolical cultists to write their spell books (usually in books of holy texts, to throw off people reading it).