Hm, so I was right partially. They are demon marks that have been taken off of demons and transplanted onto a human's body. Though they didn't really detail how, I'm sure it won't leave the demon alive after such a procedure.
So far, I feel like Dan's power seems to be only thanks to his equipments. But based on what Zenovia said, he was strong even before finding the special equipments.
I hope we can see more of it.