"Any "justice" taken too far is it's own evil. There are ties when even justice can drive people into Madness...as it happened when Shinigami, myself, and the others gave birth to Asura."
The Great Old One of Power (力の旧支配者, Chikara no Kyū Shihai-sha) is one of the Great Old Ones whose affinity to madness is that of the power and a former warlord of the Eight Shinigami Legions.
The Great Old One of Power may be a nod to H.P. Lovecraft's Cthulhu, a similar fictional and cosmic entity, with an association to the capability to gain power at the cost of one's sanity. Another connection may be found in the water that surrounds the hidden chapter of the Book of Eibon, Cthulhu also being said to sleep under water.
As its numerous conversations with Black☆Star and Death the Kid insinuates, this entity has become defined by the tremendous power it possesses. However, despite its preoccupation with the concepts of power and the seemingly flippant attitude in which it distributes this quality amongst those that it encounters, the Great Old One of Power is well aware of the true complexities behind the acquisition of power and the consequences that may arise from its pursuit. Much of this wisdom has been acquired through great adversity, due largely to the part that this entity played in the devolution of Asura into the first Kishin, which ultimately lead to the creature's decision to seek permanent confinement within the Book of Eibon. It is from this prison that it resolved to evaluate those who desire to become new rulers in the absence of the Great Old Ones, determining who amongst these candidates is most worthy to fulfil the role, while making them understand that having power does not automatically equate to being just, no matter how good their intentions may initially appear.
The entity is comprised solely from a viscous dark liquid of unknown origin, which appears to remain in a constant state of flux. In spite of this perpetual motion, due to the creature's will the substance still retains a largely humanoid form, comprising of two threadlike arms and a definitive face. These defined facial features consist from two eye sockets, created from the absence of the liquid in distinct places, and a mouth formed by a tight formation of numerous tendrils. Due to the considerable quantity of the substance involved, the entity dwarfs the majority of organisms in existence to the point where a human is substantially smaller than the being's head.
The Great Old One of Power holds considerable power, Asura holding him in high esteemed when he compared Black☆Star's power upon using madness. Though powerful, he is inferior to that of Asura, whom was stated to be the strongest of the Eight Shinigami Legions by Death.
Madness Wavelength (狂気の波長, Kyōki no hachō): Being a Great Old One, he emits a wavelength similar to that of Asura in which propagates madness. According to him, his very presence drives men into madness. Like all other with madness wavelength, certain magic and demon tools are distorted in their vicinity.
- Madness of Power (電源の狂気, Dengen no kyōki): By coming to contact with his composition of dark liquid, he can unleash the full, latent power of another at the cost of the individual's sanity as well as reveal their deepest desire. Any afflicted person almost immediately succumbing to the enticing prospects of power while neglecting anything else, as they are consumed by their own dormant insanity.
Gate: The Great Old One was capable of creating a gate made up of madness in which allowed specific individuals he's chosen to travel. He used this capability while residing within the Book of Eibon and summoned Black☆Star according to the Table of Contents. Those who are not approved to go through the gate seem to be fended off with the Madness.
Bypass: One of the capabilities of the Great Old One of Power is the ability to "bypass" and nullify other magics such as the Calculation Spells cast by a groups of witches and forcibly eject out those who entered the Book of Eibon.
Around eight centuries ago, the Great Old One of Power was affiliated with the group known as the Eight Shinigami Legions. During this time, it was possible he was present when learning of the Witches in the East's demise from Asura. However, playing a role in Asura's defection to some unknown degree, he would later sever all his ties into the Eight Shinigami Legions at some point.
At some point, the Great Old One of Power encountered the Icon whom was the personification of greed and was subsequently captured by the Sorcerer. Despite having the means to escape, however, he chose to reside within the Book of Eibon as a sort of self-imposed exile, feeling a Great Old One like himself belongs in such a place.
Relinquished to the blank interior of the Book of Eibon by Noah's decree, for the sake of his own preservation in the face of an infuriated Gopher, a panicked Death the Kid encounters this particular imposing black entity amongst the pages of his prison.
Likened to Asura in regards to the sensation of its wavelength, the strange entity confirms that the similarities are present due to it too belonging to the same category as the Kishin: Great Old One. Identifying itself towards this "fragment" as one of the remaining five original Eight Shinigami Legions, after the consumption of three at the hands of Asura; the entity includes itself amongst the likes of Death, Eibon and one other of whom Kid already knows. Amused by Kid's mistaken presumption that because of its history, it was also an ally of his, the creature instead explicitly asserts that it shares no such allegiance with him. Listing the words "Order, Knowledge, Power, Anger, Fear", the Great Old One of Power explains that the very existence of the Great Old Ones drives men to madness, before drenching Kid in the dark liquid that comprises its body.
Having revealed Death the Kid's madness, he questions of what he thinks of it. He goes further to tell him that what he has is power, completely unfettered by reason or restraint. As Kid answers his questions, he senses another individual who seeks power. Wanting to put the test of the power of Kid's "Nothingness", he later creates a portal to the hidden chapter for only Black☆Star to cross. He interrupts his battle with the intention of infecting the assassin with his Madness. When Death the Kid orders him to not interfere, he is then paralyzed and the entity states he can grant power to whoever he pleases. Questioning Black☆Star of the power of seeks, he infects with him the Madness of Power also.
As a result, the assassin reveals his desire for the reason he wants power: he has no reason and just wants to be able to beat anyone no matter who they are. Reappearing after the assassin regains himself, he once again asks the nature of the power both seek. The God of Death's answer is "Ultimate Order" and the assassin's answer is the fact he wants to gain power to assist Kid in creating the ultimate order. However, the entity and questions if the notion of imposing order by force is no different then to rule by fear and asks Black☆Star what would possible be a new, deadly sin since he experienced the chapters and went further to inform him that the answer is the secret chapter itself in which they're in. With Kid learning of his status as the Great Old One governing power, he deduces the answer would be Justice. Agreeing, the entity claims any justice taken too far itself is evil and that even justice can drive people into Madness, just like Asura.
The entity informs Kid that Death himself did not create him to make the same mistakes he made. When the assassin claims that there's others with power in which he names Maka and Soul, he learns the assassin refers to those in the Sloth chapter. Seeing as they speak of "harmony", he is convinced into having faith into them and then grants them release from the book, much to the surprise of Kid. He tells him that while the Book of Eibon is a fitting place for a Great Old One like him, he is sure the God of Death does not wish to let Noah abuse his power. As he begins freeing them, he reassures a worried Kid that his friends will be fine and overrides the Calculations Spells around them to free them from the Book of Eibon.