Mercurius, also known as the King of Mercury or Karl Ernst Krafft, serves as the Fourth Heaven—a Hadou God who seized the Throne by overthrowing Nerose Satanael. Originating from Zoar, he exploited a time paradox Satanael created, atomizing the deity with Sequere Naturam. Upon ascension, his law Vive Memor Mortis - Acta Est Fabula imposed eternal recurrence, trapping existence in an infinite loop. This law birthed the concept of possibility, expanding the Throne’s dominion into a multiverse. With a Taikyoku value near 90, he reigns as the longest-ruling Hadou God. Disillusioned by his universe’s predictability, Mercurius craves the unknown to escape boredom. His motivations shift upon encountering Marie at her Twilight Beach, a realm beyond his law. Instantly enamored, he designs a millennia-spanning plan for her to kill him and inherit the Throne. Since Marie is a Transcendence god incapable of natural succession, Mercurius orchestrates Dies Irae’s events to transform her into a Hegemony god. He manipulates key events to fuel this goal. He creates the Die Ewigkeit sorcery system, granting powers via Holy Relics. As Vice Commander of the Longinus Dreizehn Orden (LDO), occupying Seat XIII, he elevates the Nazi occultists by bestowing his sorcery. Though Reinhard Heydrich—his apoptosis and sworn friend—respects him, other LDO members despise him. Mercurius saves Rusalka Schwägelin from imprisonment, teaching her sorcery, and orchestrates the duel between soldiers Lotus Reichhart and Michael Wittmann. He divides Lotus’s soul: half merges with Michael to form Machina, while the other half fuses with his blood to craft his Holy Relic Zarathustra Übermensch, later manifesting as Ren Fuji. Ren becomes a central pawn to challenge Reinhard and catalyze Marie’s ascension. Mercurius’s relationships hinge on obsession with Marie and pragmatic exploitation. He views Reinhard as both tool and companion, bonding over their desire to transcend existence. Interactions with the LDO are marked by mutual animosity. Despite godhood, fleeting compassion surfaces—freeing Rusalka or sparing Anna Maria Schwagerin—though always serving his schemes. His powers encompass all concepts and phenomena across the multiverse, unbound by time. He employs sensories (avatars like Karl Krafft or Alessandro di Cagliostro) that induce anxiety or madness in observers. His law allows universe resets at will, even posthumously, to ensure his ideal death in Marie’s embrace. Specific abilities include Sequere Naturam (condensed stellar supernova), Dura Lex Sed Lex (gravitational singularity), Aurea Mediocritas (astrological energy surge), Omnia Fert Aetas (temporal paradox erasure), and Disce Libens (multiverse-destroying black hole). His role shifts across timelines: absent in Kasumi’s route; aiding Shirou Yusa and facilitating Gladsheimr’s confrontation in Kei’s route; interrupting Ren and Reinhard’s battle to guide them to Marie in Marie’s route, though she refuses to kill him. In Rea’s route, he acts as primary antagonist: Ren defeats him in a three-way Throne conflict, and Marie embraces him as he dies, fulfilling his wish. His law reactivates posthumously, restoring the Throne for Marie. In an alternate timeline 8,000 years before Kajiri Kamui Kagura, Hajun slays Mercurius, Reinhard, and Marie. Another timeline sees them defeat Hajun, but only Mercurius survives. He resets reality to prevent Hajun’s rise, demonstrating his law’s causality alteration. Naraka later resurrects him alongside other gods in Dies Irae Pantheon. Mercurius embodies detachment and philosophical poise, speaking poetically at length. He honors promises but displays arrogance, rejecting undesired outcomes. His actions blend responsibility—expanding the Throne to include an afterlife and multiverse—with control, manipulating countless lives for his ideal end.