The realization of ultralow thermal conductivity in a well-ordered structure is crucial for crystalline materials which consider heat conduction properties to be primary in design. We report herein an extremely low (0.32‒0.25 Wm^-1K^-1) and glassy temperature dependence (300‒600 K) of lattice thermal conductivity in a monoclinic K₂Ag₄Se₃. By applying a unified theory of thermal transport, we reveal that K₂Ag₄Se₃ features a complex phonon scattering mechanism. Delocalized vibrational correlations lead to synergistic inhibition of both propagating and wave-like heat conduction through polarization transmission. Density functional theory calculations reveal that long-range correlated Se vibrations, enhanced by delocalized hole carriers, promote interlayer lattice shearing. This shearing induces dynamically competitive expressions of different orders of anharmonicity, ultimately leading to full-spectrum phonon bunching as the temperature increases. These correlated interactions cause Se anions to vibrate together as a cluster in the low frequency region, resulting in short phonon lifetimes, low group velocities, and a large scattering phase space, which ultimately suppresses both intra- and inter-band phonon transfers. Moreover, these findings have been experimentally confirmed through low-temperature heat capacity measurements and in situ Raman spectroscopy. The insights gained from this work will advance the design of crystalline materials with tailored thermal properties.