Cú Chulainn Alter originates in the Fifth Singularity, "E Pluribus Unum," manifesting as a Berserker-class Servant. Born from Queen Medb's Holy Grail wish for a "wicked monarch" combining her traits with the Celtic hero Cú Chulainn, this alteration transforms him into the calamity-class Mad King. He leads the Celts in a destructive campaign across America. This form is an artificial construct born solely from Medb's desires and the Grail's influence, distinct from his historical legend or typical Berserker manifestations. Physically, he adopts a black-and-red color scheme. His equipment features exposed chest tattoos and a spiked tail, evoking a beast-like appearance. He wears a hood and high-heeled boots crafted from the bones of Curruid, the sea monster whose remains formed his spear, Gáe Bolg. This skeletal armor fuses with his body, symbolizing his union with the monster's essence. His crimson spear emits an ominous aura reflecting his corrupted state. His personality inverts the original's cheerfulness into cold indifference. He operates as a battle machine, devoid of joy in combat and viewing fights as mere "paperwork." His sole purpose is victory, driven by a nihilistic desire to "sprint toward death" rather than survive. He shows no compassion, disregarding pleas for mercy and slaughtering enemies indiscriminately, including former allies or mentors. Though rational and capable of conversation due to his EX-rank Mad Enhancement, he engages only on battle-related matters, ignoring non-combat directives. His relationship with his Master is strictly combat-bound: he obeys combat orders without question but rejects all other interaction. In combat, he wields enhanced versions of his original abilities. His Noble Phantasm, Curruid Coinchenn, strips enemy defensive buffs before dealing massive single-target damage, with its overcharge effect boosting his own attack and defense. He retains Gáe Bolg: Gouging Piercing Spear of Carnage, a thrown attack negating regeneration and inflicting lethal internal damage. His skills emphasize survivability: Protection from Arrows grants two evasions and a defense boost; Madness of the Spirits reduces enemy attack and critical rates; Battle Continuation provides a Guts revival. Passively, Divinity and Mad Enhancement amplify damage and Buster-based attacks. Despite his Berserker class fragility, this kit enables exceptional endurance, often allowing him to solo challenging battles through strategic skill use. Key narrative moments define his role. In the Fifth Singularity, he deals a fatal blow to Rama, slaughters resistance fighters, and kills Nero Claudius after dismissing her governance ideals. His confrontation with Scáthach underscores mutual recognition of lethal capability, though he acknowledges her as his "greatest wall." He views his original Lancer self as naively "bright" and rejects any similarity. Medb, his creator, receives detached tolerance despite her obsessive affection, deemed pragmatically "an ally." His bond progression reveals subtle shifts. Initially warning his Master to avoid him lest he "gut them on instinct," he later pledges unwavering loyalty, willing to die for their cause. This shift aligns with his fatalistic worldview—seeing himself solely as a weapon to be expended.