Involved in phytocassane phytoalexins biosynthesis. Catalyzes the conversion of ent-copalyl diphosphate to the phytoalexin precursor ent-cassa-12,15-diene. ent-copalyl diphosphate = diphosphate + ent-cassa-12,15-diene Binds 3 Mg(2+) ions per subunit. Expressed in roots and stems. Expressed from 2 to 6 days after imbibition. By chitin oligosaccharide elicitor and UV irradiation. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Ent-cassa-12,15-diene is a precursor of the phytoalexins phytocassanes A-E. Phytoalexins are diterpenoid secondary metabolites involved in the defense mechanism of the plant and produced in response to attack (by a pathogen, elicitor or UV irradiation). Belongs to the terpene synthase family.