They are demons.
They also are ridiculously good-looking, and in their true form sporting a tail and wings, garnet-colored eyes and alabaster skin.
Incubi and succubi are the souls of humans who were, first, so wicked in life that they warranted infernal punishment, and second, so clever and strong-willed even in death that they managed to survive and advance themselves in the infernal realms, where basically everything is stronger than they are and wants to destroy them. Those who make it long enough or do well enough come to the attention of Prince Vanislaas, the first incubus, and are reborn as a Human soul, so corrupted with stabilized infernal magic that they are almost unrecognizable. Not well liked in Hell, and down to the last one, obsessed with getting back to the mortal plane to spread their corruption among their own former species.
Their powers include shape changing, invisibility, all manner of illusion and flight. They’re cunning almost beyond belief; they live to seduce and corrupt.