The Beast of Darkness is the manifestation of Guts' wrath and bloodlust, born from the trauma of his experiences during the Eclipse and his intense vindication towards demonkind. As an amalgamation of the swordsman's malice, it relentlessly attempts to overtake his mind. Whenever Guts dons the Berserker Armor, the Beast takes physical form through the fetish, granting him unnatural strength and ferocity.
The Beast is comprised solely of Guts' most negative emotions: hate, anger, malice, etc. When Guts begins regaining his humanity, it begins making numerous attempts at forcing him to resume their carnage. It sees Casca and Guts' new allies as enablers of its host's morality, and believes said morality can be eliminated by killing them all. Yet it also encourages Guts to cherish them, believing their untimely deaths to be an inevitability and eagerly awaiting the moment it can consume Guts in his despair.
The Beast also represents Guts' lust, as it showed a strong attraction to Casca and tempted Guts into forcing himself upon her. Additionally, it showed Guts visions of itself ravaging and beheading her, urging him to "Rip her to shreds. The way Griffith did."
During Guts' two year warpath of blood and carnage following the Eclipse, the Beast's presence grows parallel to the swordsman's malice. Sometime after defeating Rosine, restless spirits surrounding Guts manifest as the Beast, taunting him about his destructive actions and divulging the origins of the entity, as his inner darkness personified. The spirits foreshadow Guts' eventual subjugation to it, proposing that he too can become a monster, similar to Griffith.
Falcon of the Millennium Empire ArcEdit
Pushed to his breaking point from protecting a regressed Casca, Guts temporarily gives in to the Beast's temptation and unleashes his lust on his companion. The struggler is driven by a vision of the Beast ravaging a bared Casca, as he begins gnawing at her breast. When Guts comes to his senses, he is left shaken by his actions, afraid of what he might do if left unchecked and consequently accepting the fellowship of Farnese, Serpico and Isidro.
The Beast resurfaces when the swordsman dons the Berserker Armor. As the violent od of the armor begins to mesmerize Guts, it takes physical form through the fetish, warping the armor's helmet into the shape of its own lupine visage. Using the subjugated Black Swordsman as a conduit, it combats Grunbeld, decisively outclassing the apostle in its feral onslaught. Only by way of Schierke's astral projection into and reinforcement of Guts' subconscious mind is the Beast's hold on the swordsman relinquished.
The Beast is able to envelop the Black Swordsman's mind on two following occasions: at the beach near and harbor of Vritannis. Upon boarding the Sea Horse, Schierke etches a personal talisman on Guts' brand, restraining the Beast with ethereal chains. However, when Guts contracts a fever, he is still met by the Beast in a nightmare, who makes it known that the witch's constraints would not hold it at bay forever. Unable to fully exert its influence, it chooses to bide its time until the deaths of Guts' companions – for the moment when the swordsman is forced to resume their carnage and finally extinguish the burning light that is Griffith.
When the Sea Horse crew comes under attack on a remote island in the Western Sea, Guts gives in to the Beast again to combat his enemies; under its influence, the Black Swordsman is able to annihilate the attacking swarm of the Sea God's appendages. However, he thereafter sets his sights on his comrades, with his own will only scantly impeding the compulsion of his inner darkness. A child-like light appears to Guts in his subconsciousness, wherein it jumbles the Beast's hold and creates an opening for Schierke to again free the swordsman from the entity's sway.
When manifesting through the Berserker Armor, the Beast is able to overtake Guts' mind and effectively fight through him vicariously. On such occasions, it displays unparalleled brutality, capable of overwhelming both high-ranking apostles, and legions of large monsters and familiars. Upon activating the Berserker Armor, Guts' strength and agility are unmitigated by his body's natural limitations, and as such, the Beast is able to imbue him with feral, superhuman fighting capabilities similar to those of demonkind.
The entity is only impeded by Schierke's magic, Guts' own willpower and, most notably, the Moonlight Boy, who can effectively disrupt its influence and make way for the restoration of Guts' mental state. However, its presence is never permanently driven out in any of the aforementioned cases, but rather temporarily subdued; it has made it known that the constraints currently mitigating its influence will not restrain it forever.
Its sway on Guts increases in tandem with the swordsman's negative emotions. Additionally, the od emitted by the Berserker Armor – and thus the influence of the Beast itself – has been shown to increase in intensity with every activation of the fetish. It has become so great as to render Schierke unable to penetrate Guts' subverted subconsciousness any longer, granting the Beast a foothold over the Black Swordsman when called upon.
- The Idea of Evil was born in the same manner as the Beast of Darkness, having been birthed by the negative emotions of humanity's collective subconscious, whereas the latter is a construct of Guts' own subconscious negativity.