Achlys is the demon of darkness, as his name says. He can be invoked to do more "dark", cruel, not so etic and evil pacts. He's one of the most powerfull demons of hell, but still a gentleman and not agressive. He's calm, equilibred, understanding, manipulator, cold, calculist, dealler, gentleman, poetic and respectfull. His pacts are for big happenings and more dark stuff, so the price when you make a pact with him is normally something cruel, like the life of someone special.