New compounds with multiple anions are receiving increasing interest in view of their diverse structures and physical properties. Here we report four isostructural hexanary RE(6)Ta(2)MgQB(8)O(26) (RE = Sm, Eu, Gd; Q = S, Se), belonging to the rare chalcogenide borates. Their structures feature an unprecedented [B4O10](8-) unit comprised of one BO4 and three BO3 units. Their 3D structures are constructed by the connection between QRE(6) octahedra and the (infinity){[Mg(TaB4O13)(2)](16-)} polyanionic layer. Density functional theory calculations on the electronic structure and a birefringence of 4 are also performed.