Hidden Density-Wave Instability in the Trimer Ruthenate Ba_{4}Ru_{3}O_{10}

We report an unprecedented hidden density-wave instability in the trimer-based ruthenate Ba_{4}Ru_{3}O_{10}, previously regarded as a purely antiferromagnetic insulator. This instability develops in two distinct stages: an electronically driven reconstruction at T_{A}=100  K manifested in structural, thermodynamic, and transport anomalies that remain remarkably insensitive to magnetic fields up to at least 14 T, followed only at much lower temperatures T^{*}∼20  K by the emergence of strongly nonlinear transport. Below T^{*}, charge conduction exhibits distinct depinning thresholds, sharp negative differential resistance, and unusually slow collective dynamics in the Hertz range. Direct measurements show that Joule heating is negligible, and all nonlinear signatures vanish upon only 3% Ir substitution for Ru, demonstrating the intrinsic origin. These results identify Ba_{4}Ru_{3}O_{10} as a rare correlated system hosting a strongly pinned collective electronic mode intertwined with antiferromagnetism.