The Griffiths phase (GP), a state of magnetic disorder persisting above the Curie temperature $\left(T_C\right)$, is frequently observed in rare-earth and transition-metal oxides, often leading to anomalous quantum critical fluctuations near $T_C$. In this study, we investigate the ordered double perovskite ${\mathrm{Er}}_2{\mathrm{NiMnO}}_6$ (ENMO), uncovering similar critical instabilities through detailed magnetic, magnetocaloric, and heat capacity measurements and analysis. The derived critical exponents near the critical region, $T_C$ $[\alpha =-0.942(1),$ $\beta =1.081\left(8\right),$ $\gamma =0.765\left(6\right), \text{and} \delta =1.731\left(7\right)]$ deviate significantly from all known universality classes of magnetism, yet they satisfy the magnetic scaling equation of state. Complementary magnetocaloric effect (MCE) analysis confirms these critical exponents while revealing a substantial low-temperature magnetic entropy change and adiabatic temperature values ($-\mathrm{\Delta }{S}_{\text{max}}$ $\approx$ 15 J/kg K and $\mathrm{\Delta }{T}_{\text{ad}}$ $\approx$ 15 K for $\mathrm{\Delta }H=70$ kOe), further supported by heat capacity data. The anomalous critical behavior is attributed to quantum GP formation above $T_C$, consistent with predictions from both the Bray model and Yang-Lee zeros theory. Additionally, ENMO maintains a robust insulating character across the measured temperature range, well-described by polaron hopping mechanisms. These findings challenge conventional models of magnetic phase transitions in disordered systems, emphasizing the necessity for new theoretical frameworks to account for clustered-phase dynamics and the development of revised universality classes to explain such unconventional critical phenomena.

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• High magnetic fields
• Magnetic phase transitions
• Magnetization dynamics
• Ferrimagnets
• Magnetic insulators
• Rare-earth magnetic materials
• Strongly correlated systems
• Magnetization measurements
• Resistivity measurements
• Specific heat measurements
• Susceptibility measurements
• X-ray diffraction