their true form isn't necessarily physical, but they can be killed generally by other demons. They invest their spells into there bodies and the collective, you cant simply kill them with a shot to the head, to many contingency magics in place. But if you strip them of their magic they can die. Average life span of a demon before magic is like 180 years, each time the shape shift they reset their cellular clocks, and they can shift at will into anything. none of them remember their true form and or have their true dna any longer. Though the original samples are stored in the collective.
They destroyed the everafter in a magic war with the elves and now they are stuck in it unless they are summoned out in some fashion. If they can devise the spell and can gather the energy they can do it. Females hold more energy then males since they get pregnant, only two females left. The rest are male. They use souls of other creatures as energy as well so they collect them.
Extremely fast strong and resistant to damage. In the last book we learned there original forms were not good at combat, so when the elves betrayed them they altered there forms into what they call warforms and kind of just left it that way for thousands of years. Think contingency magic, they probably can do thaumatugy on the fly, like it was evocation, I think someone mentioned like fey queens, maybe the youngest one.
Thats what rachel did early on she worked around them found ways t o bind them. They cant remain in the world when the sun is up. As or own abilities progressed she has been able to fight them but shes not 5000 years old, and though she can by far hold more energy then any of them her skill just isn't up to par and she doesn't store magic in her body. When they do that they can call up the spell with a thought instead of a cast.( she doesn't have a war form like they do i suspect the warform is why they are faster and stronger.) I think that is the source of the extreme power, Investing magic into there own bodies like a movable ward with lots of stuff weaved into it to cover things that could happen. And they have had 5k years to think of things that could happen.
And they make enchanted items up the wazo forgot about that.