Involved in the biosynthesis of labdane-type diterpenoid including marrubiin and other labdane-related furanoid diterpenoids with potential applications as anti-diabetics, analgesics or vasorelaxants (Probable). Terpene synthase the catalyzes the conversion of peregrinol diphosphate to 9,13(R)-epoxy-labd-14-ene, from (+)-copalyl diphosphate ((+)-CPP) to miltiradiene and from 8-hydroxycopalyl diphosphate (LPP, labda-13-en-8-ol diphosphate) to manoyl oxide (PubMed:24990389). peregrinol diphosphate = (13R)-9,13-epoxylabd-14-ene + diphosphate (+)-copalyl diphosphate = diphosphate + miltiradiene 8-hydroxycopalyl diphosphate = (13R)-manoyl oxide + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Present in both leaves and flowers, with higher levels in leaves. 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.