John Gauis’s eyes were described as chthnoic in HtN.
Chthonic refers to the Greek deities associated with the Underworld (or agriculture). The Furies are considered chthonic Gods; Alecto was one of the Furies. John Gauis’s eyes were Alecto’s eyes originally. So his chthonic eyes came from the chthonic figure Alecto, who was entombed in the Ninth, the House most associated with the Underworld.
Also, of course, the Greek word chthon just means 'earth'. So Alecto's eyes are chthonic also just because they are the eyes of the Earth.
Which means that when John's eyes are described as "chthonic" in HTN, amidst all the descriptions of Lyctors having their cavalier's eyes...

















