Involved in diterpenoid biosynthesis (PubMed:26077765). Catalyzes the conversion of all-trans-geranylgeranyl diphosphate to ent-8alpha-hydroxylabd-13-en-15-yl diphosphate (PubMed:26077765). ent-8alpha-hydroxylabd-13-en-15-yl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + H2O Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.