The ultrahigh sintering temperatures required for high-entropy borides (HEBs) pose a significant challenge to their processing and practical application. This study introduces an efficient low-temperature fabrication route for dense HEB-based ceramics using reactive ZrSi₂-assisted heavy direct current sintering, with a maximum heating rate exceeding 3700 °C/min. A porosity of 1.60%±0.61% can be achieved at a sintering temperature of 1000 °C, which is reduced by 600–1000 °C compared to state-of-the-art spark plasma sintering (SPS)/field-assisted sintering technique (FAST) processing of HEBs. Microstructural analysis revealed interdiffusion between HEB and ZrSi₂, leading to a core–shell HEB architecture and layered high-entropy silicides (HESs). Meanwhile, dislocations and nonuniform stress fields were observed within the HEB grains. These microstructural features synergistically inhibit crack propagation and promote crack deflection and branching. Consequently, both flexural strength and fracture toughness (K_IC) are significantly enhanced. A flexural strength of 963 MPa was attained at 1400 °C, and a K_IC of 7.4 MPa·m^1/2 was achieved at 1500 °C, surpassing most reported HEB-based ceramics. These results demonstrate that reactive ZrSi₂-assisted heavy direct current sintering is a profoundly effective approach for the low-temperature manufacturing of high-performance HEB-based ceramics.