Involved in the biosynthesis of delta-cadinene. (2E,6E)-farnesyl diphosphate = (1S,8aR)-delta-cadinene + diphosphate Binds 3 manganese or magnesium ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.