"Baenids existed from the Jurassic to the Eocene, and many species are known from throughout Hell Creek, including the charismatically named taxa Gamerabaena (Gamera Turtle) and Stygiochelys (Hell Turtle)."
― Saurian: A Field Guide to Hell Creek
Stygiochelys was a baenid testudinatan known from the Hell Creek Formation. Like most baenids, it was probably a small aquatic creature, feeding on small invertebrates.
In Saurian[]
There is no art concepts or any known plans for the inclusion of the Stygiochelys in the game. However, this testudinatan is mentioned in the official game's artbook.