Demon models can be used in AoS and 40k, that faction has always been unique that way. Demons are basically 2 armies in 1, kinda. I don't play 40k but from what i've heard they're kinda mediocre. They are getting their own codex before christmas, and that is almost bound to make them better. Fun fact: Soul Grinder This Demon unit was meant to only be a part of 40k, that is why it is so mechanical and it even has a laser cannon as an arm. The higher ups at GW demanded that this unit was also implemented into fantasy (now AoS) even though it looks completly out of place in a fantasy setting.