Dark Spear

Dark Spear (다크 스피어) is a human-made replica of nobles' soul weapon. Originally created by the Union it has been wielded only by Frankenstein.

Frankenstein use the dark energy of the Dark Spear even when the weapon is not summoned. He mainly use the dark power for his signature tentacle style attack. 

Overview: 

The Dark Spear materializes from air like a Soul Weapon does, but unlike a Soul Weapon, the aura it gives off is extremely sinister.

Frankenstein has been shown to have mastered considerable control over the weapon, being able to forge powerful items from its dark matter and to use it in battle; his control is not complete.

For example, the weapon sometimes manifests its own will and tries to consume and devour its master.

However, Frankenstein uses this nature of weapon to defeat his opponents, e.g. when he handed the Dark Spear over to the 9th Elder so it could devour him. Or piercing his opponents with the weapon e.g. Ignes and Haydn.

(For more details check out: Frankenstein's best trolling moments)

History:
The Union thousands of years stole Frankenstein's hypothesis and research on creating a weapon which can rival the soul weapon used by the Nobles .  To create the weapon known as Dark Spear it would require a immeasurable quantity of human souls. Hence Frankenstein stopped his research on it but unknown to him the Union has taken away his data and secretly started forging the weapon.

So the Union staged wars between humans and used them to harvest souls.  Similarly they created epidemics to further harvest souls.

Ultimately Union was able to create the Dark Spear but later they realized that the weapon was unstable: it would devour its wielder and anyone who was within its vicinity .

Dark Spear in its original form

So the Elders had no choice but to request Frankenstein for his help in controlling it as the weapon was built on his research and hypothesis.

Frankenstein helped them control the weapon after realizing that the weapon might devour human kingdoms if it wasn't tamed.

Frankenstein taming the Dark Spear 

Afterwards the greed of Elders increased and they wanted more.

Realizing the truth Frankenstein took away the Dark Spear with him and destroyed information on its creation so that no more similar weapons can be created and thus preventing more humans from dying.

Dr. Crombell:

Later on, Union tried to create similar weapons like Dark Spear but failed since they lacked the required data. Ultimately the Union stopped all research on this project and instead focused their attention on creating more stable and advanced weapons like the attack satellite.

Many centuries later, Dr. Crombell found research notes of Frankenstein and used them as a base to modify himself and get the dark powers. Crombell sacrificed countless lives (in the form of many small conflicts over a large span of time) to get the power while avoiding the suspicion of Union.

Unlike Frankenstein , Crombell's dark powers are stable.