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 that produces ent-kaurene from ent-copalyl diphosphate (ent-CPP) (PubMed:24990389). ent-copalyl diphosphate = diphosphate + ent-kaur-16-ene Binds 3 Mg(2+) ions per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. Present in both leaves and flowers. 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.