‘The descent into Avernus is easy’: easy is the descent into hell. Chiefly figurative: it is easy to slip into evil or immoral ways.
Avernus was the name of a deep lake near Puteoli, the reputed entrance to the underworld.
Mid 16th century. From classical Latin facilis dēscensus Avernī (Virgil Aeneid 6. 126, in some manuscripts; the preferred reading is Avernō), lit. the descent of (or to) Avernus (is) easy from facilis + dēscensus + Avernī (or Avernō), genitive (or dative) of Avernus.