Looks like we got some insight into the way chuuni black mage thinks. That's not enough for somebody like me to get a grasp on her personality, but at least we know her thought patterns are very different from those of an average adventurer.
Wait, they just left it? un-guarded?
sure the chains will prevent it from regenerating or escaping but
shouldn't they atleast think about a scenario that someone might free it?
Well, there are basically two scenarios: either they come back, and she's still there, and they take her with them. Or they come back, and she isn't there, and they are unlucky, but at least they did not have to carry a powerful monster around with them when fighting the dungeon's most dangerous monsters that guard the artifact.