Invincibility field

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The invincibility field is an aura given to Demon King Ssulal by Planner in order to make the demon king invincible to attacks by the protagonist race. This invincibility field was incorporated into the blood of the demon king, and since Dark Lords get their powers from the blood of the demon king, they also received invincibility fields (this was incorporated retroactively, so dark lords older than Ssulal, such as Kayblis also obtained an invincibility field).

In game, this ability was first shown in Rance III when dark lord Satella deflected an attack by use of an invisible orb around her. This original orb concept of the invincibility field has changed a lot since Rance III and has since become a skin tight aura around the dark lords. As stated earlier, this aura around them make them invincible to all attacks with a few exceptions.

The invincibility field doesn't block attacks that come from other Dark Lords(which means they can hurt each other), neither it works against beings that are outsiders of the Planner Scenario, more specifically, Gods and Devils.

Also, notice that the definition of "attack" used to define what the invincibility field will block or not does not seem to perfectly match today's current definition of attack. For example, it is actually entirely possible to sexually assault a dark lord because the invincibility field does not consider a sexual assault an "attack"; though this is normally a terrible idea because dark lords are generally extremely powerful entities with either extreme physical or magical prowess.

Counters of the Invincibility Field
As stated earlier, gods(including angels) and devils can ignore the Dark Lord's invincibility field. The hero of the era can also acquire the ability to pierce through the IF using the hero's blade Escudo, but at least 30% of humanity needs to be dead for him to acquire said power.

The only way for regular humans to shatter the invincibility field is by using one of the two existant "anti-demon" swords. The Demon Blade Chaos and the Holy Katana Nikkou. These two weapons can not only harm the dark lords but also shatter the invincibility field, leaving them exposed and vulnerable to attacks from other sources for a short while.

Other Attempts to Break the Invincibility Field
While humans cannot normally pass through the invincibility field, a few technologies have been created to attempt to do so with mixed results.

The Toushin were powerful mages turned into machines by M.M Rune in order to fight the dark lords during demon king Gi's reign. These machines fought on par with the dark lords, but did not have the ability to pass through their invincibility fields, making them only able to fatigue the dark lords.

A successful story comes from a technology invented by a human named Pi-R. He managed to get pass the invincibility field and extract the source of the dark lord's power, the Demonic Blood Soul. However, instead of using this incredible technology to allow humans to battle dark lords on equal footing, he decided to consume the Blood Soul himself, overcoming the will of the previous dark lord, Pi-R becaming one himself.

Another technology was created to defeat dark lords: the Red Eye. Before it could be tested on a dark lord, demon king Nighcisa caught wind of it, found it and gave it his blood, turning it into an insane dark lord. It is not certain if the Red Eye would have been able to break through the invincibility field before turning into a dark lord, but it should have been quite powerful, considering it is one of the few beings in history to have a level 3 magic skill.