Arthur Pendragon ascends to kingship of Camelot at sixteen, his claim secured two years prior when he pulled the holy sword Excalibur from a stone after it fell from the sky. This act thwarted deception attempts by his adoptive brother, Kay Pendragon. Initially, Arthur exhibits a polite, warm, and resolute demeanor, diligently upholding his royal duties with hope, despite Kay's envy and hostility. He persistently sought reconciliation, regularly visiting Kay during imprisonment and advocating for his release. Trained by Merlin of the Seven Deadly Sins, Arthur becomes a capable combatant, wielding Excalibur with exceptional skill. The sword grants him the knowledge and mastery of all previous wielders' swordsmanship, significantly enhancing his speed and strength. Prior to gaining Chaos powers, his power level measured 40,000 total: 37,600 in magic, 900 in strength, and 1,500 in spirit. He also wields Sequence, a self-repairing longsword, and exhibits immunity to certain demonic Commandments. His early armor is gold-colored steel with a horned helmet and cape.

A pivotal shift occurs when Merlin awakens Arthur as the vessel of Chaos, designating him the rightful King of Chaos. This follows the traumatic loss of his right arm to Cath Palug. Initially, Arthur unconsciously wields reality-altering magic through Chaos, struggling for conscious control. After overcoming Cath Palug in a second confrontation, he consciously manifests Chaos power to absorb the being, fully embracing his role. This grants him immense regenerative abilities, reality-altering magic, and the capacity to bestow magical powers upon his Holy Knights; these gifts include lethal "Curse Reversal" enchantments to enforce secrecy. He later forges Carnwennan, a Chaos-created sword that regenerates its blade if broken. Physically, his Chaos form alters his eyes to black sclera with purple irises.

Sixteen years later, Arthur's personality hardens into tyranny. Now thirty-two, he rules Camelot fueled by witnessing his kingdom's destruction and humanity's suffering inflicted by non-human races during the Holy Wars. He harbors deep-seated hatred for demons, goddesses, fairies, and giants, rejecting peaceful coexistence and believing humans will always be subjugated. He initiates a campaign to eradicate all non-humans from Britannia, invading territories, forcibly relocating humans to Camelot, and imprisoning or "re-educating" dissenters. He shows little regard for his knights' lives, viewing their deaths as acceptable sacrifices for his goal of a human-only world. Physically, he appears taller and more mature, wearing deceptively light floral-patterned attire and a dark blue cape. His lost arm is replaced by a dark armored prosthetic bearing Cath Palug's symbol. His relationship with Kay remains severed, though Arthur permits Kay to remain in Camelot.