Devils (悪魔, akuma) are a mighty race that inhabit Gensokyo, mostly in the Makai.
Like magicians or vampires. It was once said that they lived in different parts
of our world until mass appearances of Demon Hunters and other Christian Zealots who hunted them to extinction.
Vampires are considered to be a type of devil in Gensokyo. Youkai or Oni might be some kind of devils as well.
In some doujins, Devils are strong enough to take down a Dragon, since they once attempted a invasion on Gensokyo, but the Dragon made the Hakurei Border with the first Shrine Maiden of the Hakurei family (possibly Reimu Hakurei's ancestor) to protect it from them.