Lewis & Short

Ăchĕrontĭcus, a, um, adj., belonging to the Acheron or the Lower World: stagna, Prud. Cath. 5, 127: libri, sacred books, written, according to tradition, by the Etruscan Tages, prob. relating to the Acherontian rites of the dead, Arn. adv. Gent. 2, p. 87; cf. Serv. ad Aen. 8, 398; and Müll. Etrusc. 1, p. 77.