Ultrafast nonradiative bright-to-dark exciton relaxation facilitated by excitonic effects at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>As</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>SnS</mml:mi> <mml:mn>2</mml:mn

Momentum-forbidden dark excitons with suppressed radiative recombination and extended lifetimes are promising for optoelectronic applications. However, the relaxation mechanism from bright to dark excitons, as well as the influence of excitonic effects and temperature, remains unclear in van der Waals heterostructures. Here, the bright-dark exciton transitions of As/${\mathrm{SnS}}_{2}$ heterostructures are investigated through nonadiabatic molecular dynamics combined with the $\mathit{GW}$ plus real-time Bethe-Salpeter equation. We discover that high temperature induces phonon excitation to facilitate exciton relaxation, but the large bright-dark exciton energy difference induced by interfacial interaction limits the phonon scattering efficiency. In contrast, the electron-hole Coulomb interactions from excitonic effects exhibit strong nonadiabatic coupling and provide additional relaxation channels that dominate the ultrafast (\ensuremath{\sim}200 fs) bright-dark exciton transitions at different temperatures and stacking configurations. Neglecting the excitonic effects results in much slower transitions, with the maximum difference in transition time reaching a factor of $17\phantom{\rule{0.16em}{0ex}}292$. The resulting dark excitons exhibit long nonradiative lifetimes (\ensuremath{\sim}50 ps at room temperature). The proposed physical mechanism is crucial for understanding the formation of dark excitons and for designing optoelectronic devices with long-lived excitonic states in van der Waals heterostructures.