He isn't doing anything. He doesn't have any power to conceal himself. It's a necklace that is doing. Astiroth doesn't have worry about anything relating to his power and it's not an illsuion. It alters his form. The necklace allows him to physically alter himself to take on human traits.

Kolonis would be able to see his aura if they were trained to see it but other than that, there is no illusion.