Coulomb interactions underpin the formation of excitons, yet their role in nonequilibrium exciton relaxation has remained unexplored. Here, using first-principles real-time Bethe-Salpeter formalism combined with nonadiabatic molecular dynamics, we study the ultrafast relaxation of optically bright excitons into momentum-dark states in monolayer ${\mathrm{WSe}}_2$. It is demonstrated that Coulomb-mediated couplings between excitonic configurations are strictly forbidden in a perfect lattice but are dynamically activated by phonon excitation. This activation enables the long-range Coulomb interaction to dominate exciton relaxation, reshaping kinetic pathways and surpassing conventional electron-phonon scattering. These findings reveal a new paradigm in which Coulomb interactions actively govern nonequilibrium exciton kinetics, while phonons act as a dynamical switch that unlocks many-body relaxation channels.

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• Carrier dynamics
• Excitons
• Bethe-Salpeter equation
• Many-body techniques