The strong Coulomb interaction in two-dimensional transition metal dichlacogenides gives rise to tightly bound excitons, which dominate their optical properties. Because of their complex quasi-particle band structure, TMDs possess a variety of optically bright and dark excitonic states. Here, we present a theoretical framework to efficiently describe excitonic intervalley physics. Within this model, we first calculate the coherence lifetime and optical line shape under influence of phonons. Secondly, we present the time-resolved formation and thermalization of bright and dark intra- and intervalley excitons. We find that momentum-forbidden dark excitonic states play a crucial role in tungsten-based TMDs.