The third part of the stars in Rev.12, are the fallen Angles Lucifer drew away with him. These are what we call demons, and evil spirits.
I believe the greater amount of demonic activity during Christ first advent, birth to crucifixion. Probably had two parts, one to show Christ authority over satan, and satan's counter attack of sorts on Jesus ministry, which backfired terribly.
As someone stated, not much mention was made in the Old Testament, and only a few times following Christ ascension in the New Testament. I have found only a few, very few!, mentions of demons in any of the Early Church writings.