What is the underworld squad?did I miss something?

The squad is a fan-made reference, but it’s pretty much the residents of Chthonia.

Thanatos, Hypnos, Hecate, Nyx, Charon, Tisiphone, Alekto, Megæra, Menoetes, Askalaphos, etc.

They appear in the latest chapter of The Good Counselor.