Rechargeable magnesium batteries (RMBs) are promising candidates for sustainable energy-storage technologies, yet their advancement has been constrained by the scarcity of high-performance cathode materials. While conversion-type cathodes circumvent the structural limitations of Mg-intercalation mechanisms, most existing conversion cathodes depend exclusively on the redox activity of transition metal cations, whose capacity is inherently restricted by the reversibility of metal-ion oxidation states. Herein, we report amorphous CoS₂ hollow nanoleaves (a-CoS₂) derived from cobalt-based metal-organic frameworks, which host a dual redox Mg-storage mechanism wherein sulfur anions and cobalt cations undergo simultaneous and reversible redox reactions. Coupled with the abundant anionic sites, hollow architecture and amorphous structure, the a-CoS₂ cathode delivers a high reversible capacity of 195 mAh g^-1 at 50 mA g^-1, remarkable rate capability of 76.1 mAh g^-1 at 1.0 A g^-1, and outstanding long-term cyclability (74.6% capacity retention after 300 cycles at 200 mA g^-1), which are significantly outperforming amorphous CoS (a-CoS) and crystalline CoS₂ (c-CoS₂). Mechanistic studies confirm the reversible reconstruction of S–S bonds during magnesiation/demagnesiation, along with valence evolution of cobalt ions. This work establishes an anion-activated dual redox chemistry that transcends the limitations of conventional cation-only redox paradigm, providing new design principle for high-capacity cathode materials in multivalent-ion batteries.