Dark Ruins is were you fight dhoulmagus. To enter you must go to argonia to find a mirror and retrive it. The only way the king will give it to you is if you take his son to the royal hunting grounds and defeate multiple argon lizards till he finds a heart that is big enough. When you have the mirror, you find out that it has lost its powers. Go west to the old mans house and he will tell you that you need to find a sea dragon and use the mirror when he flashes. Then return to the dark ruins and put it in the pedistal before the entrance and it will allow you to walk the ruins.