The Beast of Darkness, often referred to by fans as the Hellhound, is a creature that periodically appears to Guts. It taunts him, questions the morality of his quest for revenge, and urges him to commit increasingly heinous acts, most notably attempting to persuade him to rape Casca. Initially, it seems like a demon or evil spirit, yet it also manifests inside Godo's cave, a place where evil spirits cannot enter. This suggests that it might be a hallucination or a part of Guts' psyche.
The Hellhound could represent Guts' own rage and hatred towards the God Hand, Apostles, and especially Griffith after the Eclipse. Among its desires, it wants Guts to kill Casca so he can return to his pursuit of vengeance. In an earlier chapter (Volume 5, Chapter 3: Wind of Swords), Casca angrily called Guts a "mad dog," which likely inspired the Hellhound's representation as his inner beast.
When Guts uses the Berserker armor, the helmet bears a striking resemblance to the Hellhound’s head. The Hellhound appears to grow stronger with repeated use of the armor but is kept in check by Guts' affection and loyalty towards his companions. These two forces are constantly at odds. In its latest appearance, the Hellhound seemed larger and more powerful than ever, bound by thick chains. However, the Hellhound has vowed that if Guts ever becomes too weak to suppress it, it will break free, take over Guts, and kill everyone he holds dear.
