There are a very small number of Angelic Origin demons.
After Lucifer fell, further Angels joined him over a period of time, and those defections still occur sporadically. Over the course of time and the acceptance of evil as their modus operandii, these demons are an elite force – the movers and shakers – almost feudal in their attempt to control the second type of demons – the purebreds.
The Angelic Origin demons cannot be killed by anyone apart from God himself and other “good” angels. It is more than the weapons that will kill them (ie angelic swords) it is the energy/power that wields the swords that is most important.