Karl Ernst Krafft, also known as Mercurius or the King of Mercury, serves as the Fourth Heaven. His origins lie in the world of Zoar, where he emerged as an anomaly within the Morning Star system, becoming the Mercurial Snake. He attained the Throne by exploiting a time paradox created by Nerose Satanael at the end of Paradise Lost, using Sequere Naturam to atomize the Hegemony God in a single strike. His reign established the world of Omega Ewigkeit, defined by endless temporal loops. He introduced the concept of an afterlife and transformed the Throne's dominion from a single timeline into an infinite multiverse, marking the longest tenure of any Hegemony God. Mercurius possesses an androgynous appearance with long dark blue hair and bears an uncanny resemblance to both Ren Fuji and Lotus Reichhart. His sensories—manifestations sharing his likeness with minor variations—often appear in different attires, though he typically wears long black tattered robes. His personality blends philosophical depth with profound boredom, driving him to manipulate events in pursuit of the unknown. This manipulative, calm demeanor and tendency for prolonged monologues provoke intense aversion in nearly all characters, particularly members of the Longinus Dreizehn Orden, who despise him despite his role as their power source and co-founder. His relationships center on two figures: Marie, whom he loves obsessively, and Reinhard Heydrich, his apoptosis and sworn friend. After encountering Marie on her Twilight Beach through a sensory, Mercurius dedicated millennia to orchestrating events enabling her to kill him and inherit the Throne, requiring her transformation from a Transcendence god into a Hegemony god. His bond with Reinhard began when Mercurius, as Karl Krafft, predicted an assassination attempt on Adolf Hitler in 1939. Their friendship deepened while forming the Longinus Dreizehn Orden, though Mercurius later admitted he initially befriended Reinhard to analyze and control his apoptosis. Despite this, their mutual respect endured, with Mercurius acknowledging Reinhard as one of the few beings who brought him joy. Mercurius engineered key events across timelines. In the 18th century, as the sensory Alessandro di Cagliostro, he freed Anna Maria Schwagerin (later Rusalka Schwägelin) from imprisonment and taught her sorcery. During World War II, he collaborated with Reinhard and Heinrich Himmler to establish the Longinus Dreizehn Orden, later sidelining Himmler. He intervened in the duel between Lotus Reichhart and Michael Wittman, splitting Lotus's soul to create Machina and fusing the other half with his blood to form his Holy Relic, Zarathustra Übermensch—later revealed as Ren Fuji, whom he crafted as a "gift" for Marie. His influence permeates all routes of Dies Irae. In Kasumi's route, he remains absent until the epilogue. In Kei's route, he transports Kasumi and Shirou to Gladsheimr, aids Shirou with Rusalka's relic, and clashes with Reinhard over halting the Transmutation of Gold. During Marie's route, he interrupts Ren and Reinhard's final battle to explain his goals, but Marie refuses his plea for death, forcing him to remain as her guardian. In Rea's route, he grants Ren full access to mercury blood powers. After Ren and Reinhard reach the Throne, Mercurius battles both but ultimately dies in Marie's embrace after declaring Ren the victor. His Law triggers a final reset to ensure Marie inherits the Throne. Eight thousand years before Kajiri Kamui Kagura, Mercurius, Reinhard, and Marie are slain by Hajun. An alternate timeline shows them defeating Hajun at the cost of all but Mercurius, who resets existence to prevent Hajun's rise. His Law, Vive Memor Mortis - Acta Est Fabula, enforces Eternal Recurrence—resetting all creation upon his death unless Marie embraces him as he dies. This rewinds even the Throne and restores erased existences. Loopholes allow timeline manipulation, such as erasing factors leading to Hajun's birth. As a Hadou God with 90 Taikyoku, Mercurius wields dominion over all concepts, phenomena, and time. His abilities include creating sensories of "complete nothingness" that induce anxiety and madness, mastering countless sorcery styles, and inventing the Die Ewigkeit system. Combat techniques involve astrological and cosmic manipulation: Sequere Naturam condenses thousands of stars into a supernova; Dura Lex Sed Lex generates a universe-pulling singularity; Aurea Mediocritas aligns stars into an incinerating cross; Omnia Fert Aetas erases targets via temporal paradox; and Disce Libens collapses all multiversal stars into a void-creating black hole. His Holy Relic, Zarathustra Übermensch, functions both as an independent entity (Ren Fuji) and a channel for Yetzirah-level sorcery.