Look, Dark King, you may be tough, but you're not tough enough to take on the entire gang of demon slayer hashiras. These guys are tough as nails, they've got incredible power, speed, and skill, and some of them have incredible magic to boot. You may have all these fancy sounding skills for dragons, blood, lightning, death, and whatnot, but it's not going to be enough to take on this group. Besides, you say you're immortal, but is that really true? Because I'm pretty sure that the hashiras have taken down some incredibly powerful demons that were supposed to be unkillable as well. So in the end, it's just a matter of time before the hashiras wear you down and take you out.