Abstract: On 26 August 2026, a large high-altitude rock–ice avalanche occurred on the northern slope of Langtang Lirung, Nepal, triggering a complex transboundary cascade of debris avalanches, debris flows, and floods. The cascade affected more than 100 km along its propagation path, causing heavy casualties and extensive infrastructure damage in both China and Nepal. Focusing on the section from the avalanche source to Gyirong Port, China, this study integrates multi-temporal remote sensing imagery, field investigations, UAV photogrammetry, and landslide-generated seismic signals to systematically investigate the characteristics, evolution, and dynamic mechanisms of the event. The results show that the main avalanche mass consisted predominantly of granitic and gneissic bedrock, with a total volume of approximately 1.1 × 10⁸ m³ and an ice content of approximately 10%–15%. The source bedrock belongs to the Greater Himalayan block, where fault damage zones, lithological contacts, and multiple sets of discontinuities jointly formed a characteristic fault-controlled geological configuration conducive to sliding. Under the combined effects of long-term cumulative seismic damage and glacier retreat, the rock–ice mass failed at an elevation of 5,200 m. The failure process exhibited pronounced progressive and multistage characteristics. Following failure, the main mass accelerated downslope and struck the floor of the glacial valley at approximately 67 m/s in average. The high-energy impact rapidly pulverized the mass and transformed it into a high-speed rock–ice debris avalanche. Approximately 100 s after the onset of overall failure, the moving mass had fully transitioned into a highly fluidized phase dominated by rapid granular motion. Through the combined effects of frictional heating, ice melting, river-water inflow, and continuous erosion and entrainment of saturated unconsolidated material, the moving mass subsequently evolved into a debris flow and travelled rapidly over a long distance along the Purepu Zangbo–Donglin Zangbo reach. Intense lateral erosion and channel-bed incision occurred along these two river sections, with a cumulative erosion volume of approximately 5.86 × 10⁷ m³. The debris flow ultimately reached and struck Gyirong Port at a mean flow velocity of approximately 22 m/s. The study further highlights the urgent need for an integrated framework for preventing and mitigating high-altitude, long-runout cascading hazards in the Himalayan alpine region, encompassing detailed identification of hazard sources, dynamic assessment, and progressive early warning at second-scale resolution using landslide-generated seismic signals; it also proposes a reconstruction approach for Gyirong Port that addresses extreme hazard risks by moving the port back from the river and rebuilding at higher elevations within the existing locality.