Multilayer ceramic capacitors (MLCCs) play a crucial role in pulsed power applications because of their rapid charge/discharge capabilities. However, the combination of high energy density and high efficiency is the main challenge in practical applications. This study presents barium titanate-based (BaTiO₃-) lead-free relaxor ferroelectric (RFE) MLCCs formulated with 0.84BaTiO₃–0.16Bi(Mg_0.2Ni_0.2Zn_0.2Zr_0.2Nb_0.2)O₃ (0.84BT–0.16BMNZZN) and platinum inner electrodes via a tape-casting method. The introduction of the high-entropy component BMNZZN effectively enhances the relaxation behavior and local nanodomains while promoting grain refinement, resulting in a comprehensive improvement in insulation performance and energy storage performance. As a result, MLCCs exhibit excellent recoverable energy density (W_rec = 15.7 J∙cm⁻³) and ultrahigh efficiency (η) of 96.4% (@1614 kV∙cm⁻¹), simultaneously showing good temperature stability over a range of −120‒100 °C (W_rec ≈ 8.9 J∙cm⁻³ with a variation of less than ±4.85%, @1078 kV∙cm⁻¹) and excellent fatigue resistance (W_rec ≈ 9.2 J∙cm⁻³ with a variation of less than ±0.82% over 10⁷ cycles, and η greater than 95%, @1078 kV∙cm⁻¹). These findings indicate that BT–BMNZZN RFE MLCCs offer a viable solution for high-power energy storage capacitors.