The Unknown (as the name implies) are an unknown alien race and monstrous living beings that are the possibly the main antagonists of the light novel and anime series Qualidea Code.
The Unknowns are responsables for the dawnfall of humanity. Their origin is unknown since they came from another dimension and caused several attacks in mass around the Earth, but is indicated that they are machines or some kind of bio-machine controlled by beings from another dimension and is clearly seen that they are not organic. Unknowns are opponents faced by the resistence of humanity. The print spin-offs put forward the idea that they are an alien menace, and what's more are utterly hostile to humanity as a whole, attacking the world killing every life form.
Events reveal that most of the said and seen Unknowns are not really unknowns, but are artificially created unmanned devices made by humans to save humans who are being saved by real Unknowns.
The Unknowns have several type of forms, some of them can have the appearance of spaceships, towers, humanoid beings, dolls, robots, fighters, star fighters and many others.