Neuroi are the main antagonists of the Strike Witches franchise. They are a mysterious, belligerent alien race that appears every so often on Earth. They literally consume swaths of the earth to gain their power, Europe and Africa are the most affected continents. Since they derive much of their energy from consuming land, they can not travel over large bodies of water. The Neuroi are the sworn enemies of the Strike Witches.
For centuries, the Neuroi have been known by many names - Kaii and Rätsel. However at the start of the Second Neuroi War, they received the name of a mythical tribe that once lived in the Black Sea region due their recurrent emergences coming from that area.
The Humans and Neuroi made their first contact when ancient Egypt was at the height of its glory. The humans were able to seal a nest of Neuroi beneath a city. They removed the name of the city from history, as even knowing that the seal would cause it to break and the Neuroi would be free and cause worldwide havoc. When these Neuroi were inadvertently released during the Second Neuroi War, their models were revealed to be identical to the models of Neuroi already fighting in the area. The reason for this fact is unknown, but it possibly implies that the Neuroi didn't develop technologically until the Second Neuroi War forced them to do so.
The Neuroi appeared throughout history after this point, but they became a serious threat to humanity until after the invention of the magic engine, a steam-powered device which amplified a witch's magic ability many times, and the resultant rise in magic usage. It is still unknown where the Neuroi came from, what planet they are from or how they were created.
- The Neuroi have a few similar traits to the Daleks as are both a hostile alien species who are emotionless, racist, misanthropic and extremely dangerous towards humanity and the rest of the universe. Except the Neuroi can't speak and the rest of their motives are unknown for now.