Kyō Senshi no Kacchū
The Berserker Armor (狂戦士の甲冑 Kyō Senshi no Kacchū?) is a cursed suit of highly durable armor, one of several of its kind, forged by the dwarves.
It removes the wearer's subconscious limitations (including pain) which prevent them from using their full potential, thus giving them extraordinary levels of strength and endurance. The armor can temporarily fix injuries: binding bleeding wounds, reinforcing broken bones, and forcibly fixing dislocated joints, piercing the flesh with thin barbs and spikes if necessary. Rather than actually healing its wearer, the armor heals itself when pierced by powerful thrust attacks.
In addition to its extremely dangerous nature, the armor can also induce insanity and "berserker rage" by drawing out the wearer's fears and therefore dissolving the differentiation between friend and foe (hence the armor's name). It can also cause chronic mental deterioration (for example, Guts' Beast of Darkness, which becomes stronger as time passes).
Upon inducing berserker rage, the armor's helm seems to take on the appearance of the wearer's inner beast. In Guts' case, the helmet and bevor are shaped like the Beast of Darkness' head and can move and function as a mouth, capable of biting enemies. When the armor's curse is active, the helm's eye sockets glow red and Guts' face cannot be seen, but when he is in partial control of his sanity, Guts' face is partially visible within the helm's maw.
The Berserker Armor was originally forged by dwarves in the Age of Man, being cursed with several dark properties that would draw out and manifest the wearer's inner demons and hate. The armor found its way into the hands of the man who would become known as the Skull Knight; where he used it for centuries to combat the hordes of evil apostles he faced. Eventually however, its malevolent nature grew too much for him to contain and he trusted it to his friend, the witch Flora. In her care, it remained locked away.
It is eventually entrusted to Guts after Flora's residence, the Spirit Tree, is invaded by Griffith's Neo Band of the Hawk; who were ordered to kill her under the belief that she would later become a threat to Griffith's power. Despite Grunbeld's immense strength, even in his apostle dragon form, Guts easily withstands his attacks and lands a few hits using the Berserker Armor.