Today’s quotation is drawn from Classical Latin repeated in a medieval source. It is from Virgil, as quoted in Gerald of Wales’ Description of Wales.
Tales casus Cassandra canebat.
Tales – adj.; such, so great, so excellent, of such kind
casus – noun, accusitive case (direct object); fall, overthrow, chance/fortune, accident, emergency, calamity, plight, fate
Cassandra – the tragic Seer and former lover of Apollo, Cassandra, who was of Troy
canebat (cano, canere, cani, canitus) – verb; sing, celebrate, chant, crow, recite, play music, sound horn/instrument, foretell
Cassandra was singing of the great calamity.
Clearly, this refers to Virgil’s Aeneaid; it’s from the section on the fall of Troy.