Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Penthēĭus, a, um, v. Pentheus, A.
Pentheus, ĕi and ĕos, m., = Πενθεύς, son of Echion and Agave, grandson of Cadmus, and king of Thebes; having treated with contempt the rites of Bacchus, he was torn in pieces by his mother and her sisters when under the influence of the god, Ov. M. 3, 514; Prop. 3, 17 (4, 16), 24; 3 (4), 22, 33; Ov. Tr. 5, 3, 40; Hor. Ep. 1, 16, 73; id. C. 2, 19, 14.
Hence,