Dark teas are prepared by a microbial fermentation process. Flavan-3-ol B-ring fission analogues (FBRFAs) are some of the key bioactive constituents that characterise dark teas. The precursors and the synthetic mechanism involved in the formation of FBRFAs are not known. Using a unique solid-state fermentation system with β-cyclodextrin inclusion complexation, as well as targeted chromatographic isolation, spectroscopic identification, and Feature-based Molecular Networking (FBMN) on the Global Natural Products Social Molecular Networking (GNPS) web-platform, we reveal that dihydromyricetin and the FBRFAs, including teadenol A and fuzhuanin A, are derived from epigallocatechin gallate (EGCG) upon exposure to fungal strains isolated from Fuzhuan brick tea. In particular the strains from subphylum Pezizomycotina were key drivers for these B-/C-ring oxidation transformations. These are the same transformations seen during the fermentation process of dark teas. These discoveries set the stage to enrich dark teas and other food products for these health promoting constituents.