Involved in the biosynthesis of melanin but also various secondary metabolites containing a naphthoquinone ring. Catalyzes the iterative condensation of five CoA-linked malonyl units to form a pentaketide intermediate. THNS subsequently catalyzes the dual intramolecular Claisen and aldol condensations of this linear intermediate to produce the fused ring of 1,3,6,8-tetrahydroxynaphthalene (THN). 5 H(+) + 5 malonyl-CoA = 5 CO2 + 5 CoA + H2O + naphthalene-1,3,6,8-tetrol kcat is 0.48 min(-1) for THN synthase activity with THN as substrate. Pigment biosynthesis; melanin biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.